Please help on causes for fuel pressure drop

[RYU]

Gents,

I'm a bit stumped on this one. At the dyno, i'm seeing a pressure drop when doing a WOT ramp run. Starting fuel pressure can be anything between 35-60psi but the 2-5psi drop is consistent at any given starting fuel pressure. For example, at a standing fuel pressure of 45psi by the time it reaches redline i'll be at 42psi.

It's not all that worrying but this shouldn't be happening.

Here's all the troubleshooting/fixes i've already done.
1. Changed the fuel pump twice. First time was on a Supra Denso. This lasted about 8 yrs but noticed this fuel pressure drop (as mentioned above) and swapped to an AEM 350 unit about 6 months and still seeing it.
2. Fuel filter is new OEM Honda
3. Fuel pressure regulator is a new AEM unit
4. Fuel lines after the fuel filter are all new but the feed and source lines which are hardly visible are still the stock ones from 30yrs ago. (I really need to change those asap)
5. I've rewired the Fuel pump to it's own dedicated 30 amp relay. The source 12V is coming from the engine bay fuse box. Not sure if that's a great spot to pull ~15amps from.
6. I'm using more efficient Bosch ignition coils so in theory overall current draw ought to be lower at WOT.

I'm stumped! I'm running out of things to change. I'm at a point were i'm starting to think the AEM pump and regulator are junk but the chances of that very low considering how new they are despite AEM parts being questionable these days to begin with.

WHAT I THINK IS GOING ON:
This is a super wild ass guess on my part. I think my fuel tank has some kind of venting problem. For example, when I release the fuel filler cap I get a loud pssssshwoooooooooooooosh as air rushes into the tank. Assumably, if the tank has a venting problem then it does not allow air volume to displace the fuel used at atmospheric pressure. Like sucking on a straw from a sealed cup.

My second assumption is one of the old lines at the gas tank somewhere is clogged.

Lastly, does anyone know if the two shutoff valves on each side of the tank aid in venting of any sort? The ones with the red hoses. It's my understanding those are shutoffs for if the car flips over. What about that circular round thingy on the driver most side of my tank below? Is the gas cap itself suppose to be a venting device as well? I see there is a large approx 1/2" black hose that appears to be the vent, but that could just be for overflow of the filler?

 

[Old Guy]

I am curious what your baseline pressure is - the one that you base your injector flow calculation on? The 'typical' base line pressure would 3 Bar/43.5 psi.

What do you mean by starting fuel pressure? The fuel pressure after the pump primes; but, the engine is not running? Fuel pressure regulators can't really regulate pressure unless they have flow through them so a static pressure after priming is not really indicative of anything. Idle fuel pressure would be a better indication of starting or base line pressure.

At idle the fuel pressure regulator is typically by-passing the maximum amount of fuel back to the tank. The stock NSX return line was set up to work with a fuel pump that was running at reduced speed under low load conditions which would reduce fuel by-pass. If you have installed a mungo huge fuel pump and are running at full voltage you may be dumping huge amounts of fuel back through the return line which is creating a high back pressure for the fuel pressure regulator. This screws up the ability of the fuel pressure regulator to control to the setting. As engine load increases by pass flow drops and back pressure becomes less of an issue.

As I recall you are running throttle bodies. That gives you a rather compromised MAP measurement. Are you still running a MAP referenced fuel pressure regulator or have you switched to a constant fuel pressure with a fuel pulse width map that has been set up to work with that? In a typical throttle body system where the injectors are designed to operate with a constant pressure difference the fuel rail pressure would start low and peak out at wide open throttle. I have another car which I set up with a 38 psi base pressure. At idle my MAP is around 50 kPa and my fuel rail pressure is approximately 31 psi (38 psi - 7 psi). At wide open throttle my pressure rises to 38 psi. Based upon the limited information it almost appears like you are running a constant pressure system rather than a rising rate system.

So, need some more details
- are you running rising rate or flat fuel pressure
- what is your design fuel pressure
- how does your fuel pressure change from idle to wide open throttle
- need to know the year of your car because the Evap system design changed during production; but, I really doubt that the Evap system is the problem. The gas cap is most definitely not a venting device.

My final observation is why sweat it. If your base / design pressure is 42 psi, a pressure error of 3 psi is only causing a 2.2% error in fuel delivery and automotive fuel pressure regulators are not exactly precision devices. You should be leaving sufficient margin in your fuel maps to allow for those kind of uncertainties.

 

[Honcho]

At idle the fuel pressure regulator is typically by-passing the maximum amount of fuel back to the tank. The stock NSX return line was set up to work with a fuel pump that was running at reduced speed under low load conditions which would reduce fuel by-pass. If you have installed a mungo huge fuel pump and are running at full voltage you may be dumping huge amounts of fuel back through the return line which is creating a high back pressure for the fuel pressure regulator. This screws up the ability of the fuel pressure regulator to control to the setting. As engine load increases by pass flow drops and back pressure becomes less of an issue.

This is an incredibly astute observation. I would have never thought of it, but it makes a lot of sense. The return line is pretty skinny....

 

[Old Guy]

Not so much observation; but, experience. I had a fancy looking expensive Aeromotive adjustable anodized billet aluminum FPR on another car. It had an interesting problem. On a transition to trailing throttle the regulator would howl and chatter like crazy and the AFR gauge was indicating unexpected transitions in AFR (more than just the expected high AFR during a trailing throttle fuel cut). In the absence of other factors I attributed the AFR problems to un expected fuel pressures. I didn't have real time fuel pressure measurements. I could set the regulator to give me my baseline pressure at idle (maximum back pressure with high return flow); but, I was never confident that it adjusted pressure correctly as return flow dropped and MAP changed.

I contacted Aeromotive product support and their response was that my 5/16" return line (same size as supply line) was way too small. It needed to be minimum 3/8" and 7/16" or 1/2" would be ideal. I ended up ditching the expensive Aeromotive FPR and installing an FPR for a 1980s vintage Nissan 280Z which was designed on the basis of 5/16" supply and return lines. That worked well since I was using the matching 280Z pump (a dead ringer for the Bosch 2 port pumps from the mid seventies) so I had a reasonable expectation that the FPR should be able to handle the return flow because it was 'factory matched' with my pump. The 280Z base pressure (38 psi) was not exactly what I wanted; but, was livable. The Nissan regulator eliminated all the noise drama and based upon what the AFR measurements and my O2 correction was doing seemed to be managing the fuel flows within a reasonable tolerance.

The take aways from this experience for my application were
- some fuel pressure regulators seem to be acutely sensitive to back pressure on the return line
- a fancy billet aluminum FPR may not perform better than a zinc plated stamped steel FPR that was designed for the application.
- having a fuel pressure regulator and fuel pump that are designed to work with the fuel line sizes that you are using can be a good thing

 

[RYU]

I am curious what your baseline pressure is - the one that you base your injector flow calculation on? The 'typical' base line pressure would 3 Bar/43.5 psi.

What do you mean by starting fuel pressure? The fuel pressure after the pump primes; but, the engine is not running? Fuel pressure regulators can't really regulate pressure unless they have flow through them so a static pressure after priming is not really indicative of anything. Idle fuel pressure would be a better indication of starting or base line pressure.

My apologies. In my screwed up terminology Starting Fuel Pressure = Baseline Fuel Pressure (or Idle fuel pressure). Sorry about that.

At idle the fuel pressure regulator is typically by-passing the maximum amount of fuel back to the tank. The stock NSX return line was set up to work with a fuel pump that was running at reduced speed under low load conditions which would reduce fuel by-pass. If you have installed a mungo huge fuel pump and are running at full voltage you may be dumping huge amounts of fuel back through the return line which is creating a high back pressure for the fuel pressure regulator. This screws up the ability of the fuel pressure regulator to control to the setting. As engine load increases by pass flow drops and back pressure becomes less of an issue.

This is VERY VERY interesting and I need to explore this further, perhaps even create a larger return line. However, in my case, as engine load increases fuel delivery or fuel rail pressure drops about 2-4psi which is contrary to the theory?

As I recall you are running throttle bodies. That gives you a rather compromised MAP measurement. Are you still running a MAP referenced fuel pressure regulator or have you switched to a constant fuel pressure with a fuel pulse width map that has been set up to work with that?

The vacuum reference to the FPR has been disconnected. The FPR diaphragm is essentially open to atmosphere. I did not plug it either. You are correct, the MAP signal was available but is also too finicky and inconsistent with ITBs and which is the primary reason it's been disconnected. I'm running the AEM Infinity in Alpha N and through a Volumetric Efficiency tuning strategy. My prior HKS V-Pro was running off of Alpha N & injector pulse width.

In a typical throttle body system where the injectors are designed to operate with a constant pressure difference the fuel rail pressure would start low and peak out at wide open throttle. I have another car which I set up with a 38 psi base pressure. At idle my MAP is around 50 kPa and my fuel rail pressure is approximately 31 psi (38 psi - 7 psi). At wide open throttle my pressure rises to 38 psi. Based upon the limited information it almost appears like you are running a constant pressure system rather than a rising rate system.

Yes, it's constant fuel pressure. Definitely not a rising rate system which is what I ran many years ago with the Comptech Supercharger Fuel System.

So, need some more details
- are you running rising rate or flat fuel pressure

Flat fuel pressure

- what is your design fuel pressure

It was loosely "designed" for 40psi of rail pressure given my injectors are 1050cc's. We tested with 30psi, 45psi, & 60psi of base fuel pressure and at each level we observed various rates of pressure drop. I don't recall if 30psi had less of a pressure drop than at 60psi. It was a drop nonetheless and the logs didn't point to an obvious reason at the time.

- how does your fuel pressure change from idle to wide open throttle

At the moment, at WOT a typical example is starting base pressure will start at 45psi then gradually and pretty consistently slopes down to approx 42 psi at 7000 rpm.

- need to know the year of your car because the Evap system design changed during production; but, I really doubt that the Evap system is the problem. The gas cap is most definitely not a venting device.

My NSX is a 91 with the discontinued charcoal cannister. @MotorMouth93 brought up an interesting point yesterday. Given i've yanked out the factory ECU completely and given that the AEM Infinity is not setup to control any of the purge valves in the EVAP system... I'm wondering if the venting mechanism is essentially closed. I want to proceed carefully here since venting of gasoline fumes shouldn't be taken lightly. Any ideas? Also, i've never heard of this issue come up with folks on standalone ECUs with NSXs.

My final observation is why sweat it. If your base / design pressure is 42 psi, a pressure error of 3 psi is only causing a 2.2% error in fuel delivery and automotive fuel pressure regulators are not exactly precision devices. You should be leaving sufficient margin in your fuel maps to allow for those kind of uncertainties.

That's exactly what I thought at first. A 2-4psi drop seems like a nothing burger to me as long as the Injectors can compensate without going past a healthy duty cycle. However, virtually everyone i've spoken with, pretty much tell me there should be NO pressure drop and that it could potentially be a sign of a problem. My tuner refused to tune in VTEC until I got a hold of the fueling problem as an example. So perhaps it is an issue as what some of the more experienced folks are telling me.

Here's another problem i'm dealing with that may or may not be complementary to this problem. The front bank (cyl 4,5,6) is calling for up to an additional 17% of fuel which is super super weird. I painstakingly balanced the ITBs at various RPMs so i'm certain it's not caused by an imbalance in intake airflow. There is a chance my the rear bank catalytic converter could be clogged so I will check on that this wknd. Lastly, we swapped o2 sensors and the fuel imbalance still remains in the front bank so it can't be a faulty o2. It truly feels like there's a problem in the fuel lines, fuel tank, or venting somewhere.

Lastly lastly, I just received a clean bill of health from Injector Dynamics on the ID1050x injectors I was using. They all flow within specification so it's not the injectors. I've got many stones to uncover in my shotgun approach but i'm stumped as to what could be causing this.

Amazing feedback @Old Guy. I thank you sir!

 

[RYU]

Not so much observation; but, experience. I had a fancy looking expensive Aeromotive adjustable anodized billet aluminum FPR on another car. It had an interesting problem. On a transition to trailing throttle the regulator would howl and chatter like crazy and the AFR gauge was indicating unexpected transitions in AFR (more than just the expected high AFR during a trailing throttle fuel cut). In the absence of other factors I attributed the AFR problems to un expected fuel pressures. I didn't have real time fuel pressure measurements. I could set the regulator to give me my baseline pressure at idle (maximum back pressure with high return flow); but, I was never confident that it adjusted pressure correctly as return flow dropped and MAP changed.

I contacted Aeromotive product support and their response was that my 5/16" return line (same size as supply line) was way too small. It needed to be minimum 3/8" and 7/16" or 1/2" would be ideal. I ended up ditching the expensive Aeromotive FPR and installing an FPR for a 1980s vintage Nissan 280Z which was designed on the basis of 5/16" supply and return lines. That worked well since I was using the matching 280Z pump (a dead ringer for the Bosch 2 port pumps from the mid seventies) so I had a reasonable expectation that the FPR should be able to handle the return flow because it was 'factory matched' with my pump. The 280Z base pressure (38 psi) was not exactly what I wanted; but, was livable. The Nissan regulator eliminated all the noise drama and based upon what the AFR measurements and my O2 correction was doing seemed to be managing the fuel flows within a reasonable tolerance.

The take aways from this experience for my application were
- some fuel pressure regulators seem to be acutely sensitive to back pressure on the return line
- a fancy billet aluminum FPR may not perform better than a zinc plated stamped steel FPR that was designed for the application.
- having a fuel pressure regulator and fuel pump that are designed to work with the fuel line sizes that you are using can be a good thing

You have convinced me with this post to try the factory fuel pressure regulator in place of my AEM FPR! That will not help my oversized Fuel Pump (an AEM 350lph unit) but it's something I can get to easily. I'm dreading, I mean absolutely DREADING dropping this fuel tank again for the 4th time in my ownership. It's the absolute worst job on the NSX given I don't have infant hands.

 

[RYU]

Follow-up question if I may - Where is the optimal place to install a fuel pressure sensor?

Currently I have my Denso 150psi fuel pressure sensor on the typical NSX location which is a 1/8 NPT tapped into the output fuel line of the factory fuel filter right before the fuel rail.

I suppose I could setup a sensor rig to independently read fuel pressure of bank one vs. bank two.. however, I hesitate because this is a lot of work for just troubleshooting and given i've never heard of this being an issue before on even 600hp NSXs using the factory rails, I pause.

 

[bogle]

17% more fuel up front seems crazy! It feels like that could be a problem. 17% is a lot! But what could it be…

My car is the opposite: the rear bank runs lean and could accept probably 2% more under cruising loads, but they agree under load

I think it would be super interesting to measure pressure on the output of the rear rail. Not sure what fittings you have, but these tees exist:

RaceFlux -6AN Male to -6AN Female w/ Dual 1/8" NPT Female Sensor Ports

RaceFlux -6AN Male to -6AN Female w/ Dual 1/8" NPT Female Sensor Ports

www.anhosefittings.com

Super interested to see what the stock fpr shows. Maybe it is the regulator. Possible AEM designed the one you have to expect reference vacuum and so over various flow rates, it isn’t necessarily going to hold constant pressure. If you had a stock manifold setup and a vacuum line hooked up, I bet it would totally mask this and you’d be seeing like 50-55psi at wot

 

[Old Guy]

I don't think you have a pressure drop problem in your system. I think you have a pressure rise issue which may or may not be a problem .

This is VERY VERY interesting and I need to explore this further, perhaps even create a larger return line. However, in my case, as engine load increases fuel delivery or fuel rail pressure drops about 2-4psi which is contrary to the theory?

I don't think so. In the 1991 NSX service manual, page 11-88 it states if the fuel pressure is higher than spec - inspect for pinched or clogged fuel return hose . I think the response of the regulator is that if it sees increased back pressure on the return line the control pressure goes up. That increased back pressure can be caused by a restriction or perhaps by trying to push too much fuel down a tiny hose because of an oversized pump. Your too much fuel problem occurs at idle (causes high pressure) and goes away as engine load increases (returns to correct pressure).

However, virtually everyone i've spoken with, pretty much tell me there should be NO pressure drop and that it could potentially be a sign of a problem. My tuner refused to tune in VTEC until I got a hold of the fueling problem as an example. So perhaps it is an issue as what some of the more experienced folks are telling me.

That is a conventional response and is a correct response if you start out at your target pressure at idle and the pressure drops below the target pressure as engine load increases. That would be an indication of inadequate supply capability which causes full load fuel starvation paranoia. Your problem is different because you are starting out above your target pressure at idle and the pressure is declining to your target pressure at full load.

I am not super familiar with the 1991 Evap system. I think the only function that the ECU has is to control the purge solenoid. If there is no ECU connection to the purge solenoid that may mean that the canister is in continuous purge or it is never being purged. You would need to look at the section in the service manual to figure out how that purge valve works. I think the two way valve in the system always allows air into the tank so from that perspective you should be OK. If pressure in the tank goes high (hot day) the two way valve is supposed to open and relieve pressure by venting through the canister which traps the fumes. The only thing to consider is what are the problems with being in continuous purge. If it never gets purged the cannister will eventually get clogged and become in effective. The conditions for purge valve control 'appear' pretty simple - solenoid off after engine start up with coolant > 70 C. If that is all it is you may be able to control the valve with your ECU if you have some unused outputs. That is my 10,000 ft view of the 1991 Evap system!

I think your front - rear imbalance problem is something different and not related to the fuel pressure oddity. As my Doctor says - need to book another appointment!

I am not sure that replacing your AEM regulator with the OEM regulator will make the problem go away. You 'may' be supplying way more fuel than the OEM regulator was ever designed to handle with the result that it may also suffer from the same problem. It may not be as bad as or it could be worse than the AEM regulator. In my particular case the Aeromotive regulator seemed to be acutely sensitive to back pressure and the Nissan regulator particularly insensitive to back pressure. If you still have your original OEM regulator it might be worth a try; but, the thing to keep in mind is that the base pressure for the 1991 NSX appears to be 50 psi (page 11-95 in the SM). The change from the 40 psi to 50 psi base pressure is likely enough to force a retune of you ECU. If the pressure rise (because everybody seems to hate the term pressure drop) as the engine load drops bothers you, I would be more inclined to experiment with a larger return line which was Aeromotive's advice to me. To be effective you would have to make sure that the fitting at the tank does not present a restriction. Or, consider my next suggestion of throttling the fuel pump.

Aeromotive makes an electronic speed control for their fuel pumps. Allows you to drop speed at low engine loads and crank up the speed as engine load increases. Does AEM have anything similar. That might be a path to reducing your fuel over delivery at low engine load. Or, you could re implement the thing that everybody seems to hate, the fuel pump resistor. If you have a spare output on your ECU you could configure it to similar settings that the OEM ECU uses to reduce your over delivery problem at low engine load. A resistor and a relay and some coding versus replacing your fuel pump or trying to fit a larger return line or retrofitting the original fuel pressure regulator seems like an easier solution.

 

[bogle]

This could be normal?

Fuel Pressure Regulators - VaporWorx

www.vaporworx.com

It is normal for a non-manifold referenced pressure regulator to drop pressure in order to obtain full fuel flow
…
The C5 FFR provides a 58psi at idle fuel pressure and an approximate 52psi at full flow.

 

[Old Guy]

Follow-up question if I may - Where is the optimal place to install a fuel pressure sensor?

Currently I have my Denso 150psi fuel pressure sensor on the typical NSX location which is a 1/8 NPT tapped into the output fuel line of the factory fuel filter right before the fuel rail.

I suppose I could setup a sensor rig to independently read fuel pressure of bank one vs. bank two.. however, I hesitate because this is a lot of work for just troubleshooting and given i've never heard of this being an issue before on even 600hp NSXs using the factory rails, I pause.

If you are looking to try and diagnose a problem, then you have to instrument 'everywhere'. If you just want to monitor to look for emerging problems (clogged fuel filter or failing pump) then monitoring at the normal location at the fuel filter is just fine. I don't think fuel delivery is the cause of your 17% lean problem (if that is where you were going) so I advise against the everywhere approach.

Here's another problem i'm dealing with that may or may not be complementary to this problem. The front bank (cyl 4,5,6) is calling for up to an additional 17% of fuel which is super super weird. I painstakingly balanced the ITBs at various RPMs so i'm certain it's not caused by an imbalance in intake airflow. There is a chance my the rear bank catalytic converter could be clogged so I will check on that this wknd. Lastly, we swapped o2 sensors and the fuel imbalance still remains in the front bank so it can't be a faulty o2. It truly feels like there's a problem in the fuel lines, fuel tank, or venting somewhere.

As noted, I don't think fuel delivery is the cause of this problem. As I recall, fuel delivery is to the front rail right side and fuel return from the regulator is back rail right side. If anything, given the direction of fuel flow the fuel pressure on the front rail has to be a tiny, tiny bit higher than fuel pressure at the back rail. That does not jive with your running lean lean on the front.

Is this 17% error at low - mid load? If so, is your ECU set up with a single fuel map with separate O2 correction for each bank or do you have separate maps with O2 correction for each map? You may have a problem with the O2 sensor or the O2 feedback loop control parameters. I assume that you suspend O2 control at high load and just run open loop so O2 correction flakiness should not cause your lean condition on the front at high loads. In that case, if you have separate maps I would look for a problem / difference between the maps. On my other car, the ECU has a single fuel map; but, it also has 4 fuel trim maps (4 cylinder) that allow you you to adjust individual injector pulse widths up or down relative to the injector pulse width calculated form the fuel map. I use this trim map to adjust for the fact that My intake manifold slightly starves the #4 cylinder. If your ECU has a similar trim function maybe it is doing somethin unexpected.

 

[RYU]

Thanks so much for the feedback guys. I really really appreciate it. So much to digest here not sure yet how to respond properly.

I just got the injectors back from Injector Dynamics and they confirmed they were not the problem but now that they're back I can record the logs myself and share them here.

The logs will tell a thousand words as to the behavior, not necessarily point to a fix though.

 

[RYU]

Another quick note. I'll be blowing out the easily accesible fuel lines and rails with compressed air this weekend as a hail Mary. I won't be able to R&R the fuel tank lines at this point. I'll also swap out the High Flow Cats with Test Pipes in case perhaps the rear bank is clogged and perhaps why the front bank is calling for more fuel based on AFRs.

@Old Guy - I owe you many answers and I promise I'll get to them (THANK YOU). The AEM is tuned with AFR feedback per the individual front/rear wideband O2s. Closed Loop fuel compensation is setup individually per O2 sensor (i.e. per bank).

 

[RYU]

Lastly, the car drives perfectly smooth. No hints of misfires, timing issues, or ignition coil inconsistencies. Not sure if mentioned this btw.. but the stock motor is back in my car. This is NOT the 3.6L which is being disassembled for forensics.

 

[Old Guy]

With respect to your 17% fuel correction for the front cylinders, are you determining that value from the amount of correction being applied based upon the O2 sensor measurements? If so, you switched the sensors and the result indicated that the sensor was not the problem. When you did this, did you also switch the wideband controller (assuming the controller is not on board the ECU) or just the sensor? The reason I ask is I had an Innovate wide band controller that developed an internal problem in the heater circuit (a high resistance connection for the heater control MOSFET on the board as I recall) with the result that the controller was not accurately controlling the sensor temperature. Depending on which side of the correct temperature you end up on temperature errors can cause the O2 sensor to skew high or low relative to true. If you switched both the controller and sensor, then its not a controller issue. Some wide band controllers have LEDs which will report temperature control errors; but, I don't know that they all have this feature.

Since you are adding fuel to correct the mixture, make sure that you have not developed an exhaust manifold leak upstream or close by downstream in the vicinity of the controlling O2 sensor. If air is entering into the exhaust stream the free O2 will be detected by the sensor and the sensor will report this as a lean fuel mixture. Even if the leak is downstream of the sensor, under the right circumstances reversion can cause O2 to be sucked back and influence the sensor. Farther downstream than 6" and the reversion effect probably disappears.

 

[RYU]

With respect to your 17% fuel correction for the front cylinders, are you determining that value from the amount of correction being applied based upon the O2 sensor measurements?

Yes, exactly. The Infinity Closed Loop Lambda Feedback function seems to be fast enough to adjust in real time.

If so, you switched the sensors and the result indicated that the sensor was not the problem.

Yes, that's correct. I've got several skin burns on my arms to prove it! haha

When you did this, did you also switch the wideband controller (assuming the controller is not on board the ECU) or just the sensor? The reason I ask is I had an Innovate wide band controller that developed an internal problem in the heater circuit (a high resistance connection for the heater control MOSFET on the board as I recall) with the result that the controller was not accurately controlling the sensor temperature. Depending on which side of the correct temperature you end up on temperature errors can cause the O2 sensor to skew high or low relative to true. If you switched both the controller and sensor, then its not a controller issue. Some wide band controllers have LEDs which will report temperature control errors; but, I don't know that they all have this feature.

You sir are brilliant. No, I did not isolate if the controllers themselves are wacky, just the wideband sensors. It should be easy enough to point the Onboard Controller to the front bank (Bank 2) and the UEGO external controller to the rear bank (Bank 1). I had not thought of this. Thanks for mentioning it.

Since you are adding fuel to correct the mixture, make sure that you have not developed an exhaust manifold leak upstream or close by downstream in the vicinity of the controlling O2 sensor. If air is entering into the exhaust stream the free O2 will be detected by the sensor and the sensor will report this as a lean fuel mixture. Even if the leak is downstream of the sensor, under the right circumstances reversion can cause O2 to be sucked back and influence the sensor. Farther downstream than 6" and the reversion effect probably disappears.

This is a also a great point I had not considered. I do have a suspicion that something in Bank 1 exhaust path is restrictive. I'm installing Test Pipes this wknd and will investigate.

Another interesting thing I noticed, since you've convinced me to go down the Fuel Return Line rabbit hole is that the return banjo fitting on the AEM FPR is quite small. It's smaller than even the ID of the NSX return hard line itself by ~2mm. I've ordered a new -6AN banjo fitting and will swap that out. It was bothering me that I was using serpentine hose clamps on the return line anyway. So odd that the ID of the AEM FPR return fitting is so small.

 

[RYU]

BTW... I can confirm there is no bleed or purge valve in the tank. Well.. at least on mine. This was my experiment. Please critique it!

I ran compressed air through the return line right at the FPR location. As the air pressure built up air did not escape anywhere. Only gasoline escaped through the open fuel lines as the internal air pressure of the gas tank came to equilibrium with atmosphere. I had to remove the gas cap as that was the only way for air to escape when I tried the experiment again. I'd have though there would be a pressure valve inside the tank of some sorts?

 

[Old Guy]

Nothing is 'in the tank'. The purge valve sits on the evap cannister and the 2 way valve sits on top of the tank.

This is the diagram for the 1991 Evap system from the service manual.




The purge cut-off solenoid may or may not be in that big plastic control box in the engine compartment. I have a 2000 and it has a completely different / more complicated Evap system than the 1991 so I am not certain about location. The solenoid controls the purge control valve via a vacuum line and the purge control valve is mounted on the top of the Evap cannister.

On the 1991, the two way valve sits on top of the tank. On your car I think it is this thing



I think the black line off the two way valve is what should connect to the evap cannister.

The test as described in the service manual is a bit confusing. That red line should go to the tank body and the service manual indicates that
- if you apply a vacuum to the valve port that the red line is connected to the valve stays closed until the vacuum reaches 0.6 in Hg at which point the valve opens to allow air to enter into the tank
- if you apply pressure to the valve port that the red line is connected to the valve should stay closed until the pressure reaches 1.4 in. Hg at which point it opens and allows the tank to vent out through the black line to the cannister.

To do the factory test you need one of those little hand held Mity vac tools. 0.6 in Hg is about 0.3 psi. Using a shop air supply might have overwhelmed the two way valve.

 

[RYU]

Thanks for the info @Old Guy . Very helpful. I investigated this entire evap setup today. The purge valve solenoid (vacuum line #6) is effectively disconnected with the Infinity ECU. @MotorMouth93 mentioned he could get me the logic the stock ECU uses and possibly mimic that in the Infinity. I'm not super excited about that and I consider it a last resort.

My two way valve is interesting. It will only allow air INTO the fuel tank, at least in my setup. Isn't it suppose to be a two way valve? I don't have a vacuum pump but I was using various methods to engage the purge valves and various diaphragms. I'm going to unplug this vacuum line and see if this allows venting into the tank temporarily while testing. If this solves my fuel pressure problem I'll have to figure out how to actuate the #6 solenoid.

 

[Old Guy]

Thanks for the info @Old Guy . Very helpful. I investigated this entire evap setup today. The purge valve solenoid (vacuum line #6) is effectively disconnected with the Infinity ECU. @MotorMouth93 mentioned he could get me the logic the stock ECU uses and possibly mimic that in the Infinity. I'm not super excited about that and I consider it a last resort.

My two way valve is interesting. It will only allow air INTO the fuel tank, at least in my setup. Isn't it suppose to be a two way valve? I don't have a vacuum pump but I was using various methods to engage the purge valves and various diaphragms. I'm going to unplug this vacuum line and see if this allows venting into the tank temporarily while testing. If this solves my fuel pressure problem I'll have to figure out how to actuate the #6 solenoid.

2nd thing first. My understanding jives with your understanding. The only detail is that the valve appears to be normally closed in bot directions. When vapour pressure builds up in the tank and exceeds 1.4 in Hg the valve opens and allows the tank to burp into the Evap cannister. Once pressure is relieved the valve closes so the tank is not continuously open to vent to the cannister. That is why you sometimes get that pfft when you open the gas cap on a hot day - pressure has built up; but, not above the 1.4 in Hg required to open the valve. Same deal on air flow into the tank. The valve does not open until a vacuum of 0.6 in Hg is present which opens the valve and drops the vacuum in the tank. Same deal. The valve open and relieves vacuum and closes and then opens so it is probably never continuously open.

1st thing 2nd. Purging the cannister requires manifold vacuum to operate the purge valve on the cannister. With your ITB set up you need to evaluate whether you have sufficient vacuum to open the purge valve under the correct conditions. You might have to look for a different cannister which has an electrically operated purge valve right on the cannister and skip the separate solenoid and vacuum lines. The other option is go retro. Evap cannisters first showed up in 1970 or 1971 and the very first ones were completely passive. They had a line from the tank into the cannister plus a purge line from the cannister to the throttle - typically right at the throttle blade. The tank vented continuously into the cannister and purged continuously (or whenever there was sufficient vacuum at the port at the throttle blade). Completely uncontrolled.

The biggest consideration with either a stock evap installation or the retro refit is do you have anyplace on the ITB system where you can generate enough vacuum to to get purge airflow during the correct purge conditions? If not then don't waste your time on refitting the evap system.

 

[RYU]

Man... I'm so burnt out. I spent all weekend on this and the problem is still there. Here's a list of what I've done so far because i'm starting feel like i'm chasing my tail. This is a g'damn stock motor. Stock components should be fine, but I find myself upgrading or having used upgraded parts from the prior build.

Problems:
1. Fuel pressure drops by ~2ps from idle to WOT
2. Bank 2 (Cyl 4,5,6) requires up to 17%+ fuel (I'm more worried about this vs. the fuel pressure drop)

Tested/Repaired/Replaced
* voltage (at ECU) is good (Idle: 13.9v > wot 13.7v). Main Battery is new
* new gas cap
* Injectors verified and tested good
* swapped O2 widebands and controllers. any swap configuration shows Bank 2 needs more fuel
* rebalanced the ITBs
* installed test pipes/remove high flow cats
* verified fuel return line is not clogged
* rewired the fuel pump voltage supply wiring

What I might need to do next:
* Try Stock fuel pressure regulator or swap to new FPR
* Try new fuel pump and completely rebuild all fuel supply lines in -6AN
* Check fuel pump bypass or if pickup is too close to floor of tank.
* Cut open fuel filter and/or use aftermarket fuel filter
* Try larger fuel pump return line
* R&R the various gas tank purge valves and hoses
* Try larger fuel rail (long shot)
* Try to log with gas cap OFF
* Try new muffer and headers to verify flow of exhaust
* Compression check and Leak Down
* Swap spark plugs
* ???

Log Snapshot
This is the last log from this afternoon. The vertical purple line towards the right is the target line and represents the numerical figures at that moment in time. At this position Throttle is at 96.6% and the front bank Lambda is adding 14% while the rear bank is adding 5%. I've seen this go up to 17% and 0% respectively and this is merely the non-vtec cam lobes with moderate timing and a bit AFR rich for safety. I can't figure out why Bank 2 needs so much fuel... Perhaps there's something wrong with Bank 1.

 

[RYU]

I have a bad feeling one of the cam gears is off a tooth a new TB was put on before the engine went in

 

[bogle]

Are you running the same tune from the built motor? If so, this stuff is probably moot, but could be worth a check

Maybe an obvious thing, but curious if you’ve dug through all the fuel trims? I don’t know what the scale is on that chart, but if I’m reading it correctly, it looks like the the pink and green lines are pretty far apart even at wot, though they do follow a similar pattern. Are all the cyl-specific trims zeroed out? Is there a gain set on either of the wideband inputs in the software? I assume you are running analog 0-5v outs from the controllers into the ecu? Do the widebands have CAN outs? Might be worth reading the digital outs to see if they agree with the analog outs. If you turn off o2 correction, are things a little closer?

Another dumb thing: are the o2 sensors assigned to the correct cylinders? At least on the early cars, O2 number 1 is the front bank and o2 two is the rear...

I still think the fuel pressure drop is probably normal. I can’t imagine a dumb spring in the FPR would be able to keep the pressure perfectly stable no matter the flow rate. I guess MotorMouth would know best as he is running itbs likely without fpr reference and probably logging fuel pressure

Also curious if there is a lowish effort way to check that the timing belt is on the right teeth, please post the approach of you do check it!

 

[RYU]

Are you running the same tune from the built motor? If so, this stuff is probably moot, but could be worth a check

Definitely a new tune, but using the built motor as a "base map". I'm wondering if there's something hidden in the bowels of the tune that's causing this. I really don't want to start a base CAL from scratch... sigh

Maybe an obvious thing, but curious if you’ve dug through all the fuel trims? I don’t know what the scale is on that chart, but if I’m reading it correctly, it looks like the the pink and green lines are pretty far apart even at wot, though they do follow a similar pattern.

Yes, the scaling is off in the log parameters. So it's showing up weird. I keep forgetting to fix that.

Are all the cyl-specific trims zeroed out? Is there a gain set on either of the wideband inputs in the software?

I checked that first! All individual cyl trims are zero'd out. I'm unsure about gains on either widebands though. Worth a look tomorrow. However, I ruled out that it's a wideband issue. I went through a series of troubleshooting steps in regards to the Lambas sensors. All still point to the same issue whereby front bank is requiring more fuel.
1. swapped in two new sensors once at time same locations in the exhaust
2. swapped the two sensors between front vs. rear
3. did the above but swapped the locations and pointers to the controllers themselves. I'm on the Infinity 508 which has one onboard and one UEGO/X-series CAN WB controller. So at first the onboard was pointed to Cyl 1-2-3. X-series pointed to Cyl 4-5-6 in the software and hardware. Then ran another test with both swapped in terms of location and software config.

Unless i'm missing something, I think I can rule out a Wideband sensor/controller problem.

I assume you are running analog 0-5v outs from the controllers into the ecu? Do the widebands have CAN outs? Might be worth reading the digital outs to see if they agree with the analog outs. If you turn off o2 correction, are things a little closer?

I'm using strickly CAN on both sensors. Well.. tbh i'm not sure what the onboard uses but i'm sure it's digitally directly connected on the board so no CAN. At this point i'm too afraid to run it with the o2 feedback corrections OFF. I'd have to add at least some fuel in the individual cyl tables for 4-5-6 to feel safe about it? Unsure of how to do this safely enough

Another dumb thing: are the o2 sensors assigned to the correct cylinders? At least on the early cars, O2 number 1 is the front bank and o2 two is the rear...

Yes, to confirm, I set them up backwards. The feedback corrections where wild. They were maxed out to my limit settings. Which were add up to 20%. Remove up to 10% fuel at any given time. So one was -0.10 and other was +0.20. I quickly aborted that run!

I still think the fuel pressure drop is probably normal. I can’t imagine a dumb spring in the FPR would be able to keep the pressure perfectly stable no matter the flow rate. I guess MotorMouth would know best as he is running itbs likely without fpr reference and probably logging fuel pressure

Yes, agree. At this point i'm not even worried about the pressure dropping 2psi. It's annoying me and something my OCD will have to fix at some point or at least figure out why it's doing it.

Also curious if there is a lowish effort way to check that the timing belt is on the right teeth, please post the approach of you do check it!

This is what i'm focused on this minute. Ask me 5 mins from now and it might be something else. I just got off the phone with an old friend who use to run an Acura Dealer service shop. He has his money set on a cam tooth being off but provided no easy way to check it w/o removing the valve covers and checking the dots. Even then because i'm running MLS head gaskets and the heads had a very light decking about 8yrs ago when my HG went out.. i'm sure that because i'm still on the stock cam gears my cam timing will be a tiny bit off. The only real way is to degree the motor but virtually impossible to that while on the car. I'm going to try a leakdown and try to get it to TDC with a dial indicator. If a tooth was off my assumption would be that the leakdown will fail by virtually the same % across the entire bank. This is a pure guess on my part... I'm not even sure if at TDC, if the cam gear was off by just a tooth, if it would even crack any of the valves open. No clue! It's a long shot but easy enough to test for.

 

[Old Guy]

I think it is good that you are no longer so hung up on the fuel pressure question. That is a good approach as long as the fuel pressure is on target at high load.

I need some clarification about the data log. First off, I see two pink / purple lines and they are rather similar. I assume the top pink line is Lambda 2 and the bottom pink-purple line is the fuel correction? What are the units for the LambdaFB? You seem to be assuming %; but, could it be a ms adder? 0.14ms added to 4.3 ms would only be a 3.2% correction. It would be useful to display the injector pulse widths for both the front and rear banks. If your logging software auto scales see if you can change the scaling parameters so that

What is that fuel pressure measurement? Is there a scaling problem because 61.7 is well above the target of 40 psi.

The AFRs measurements confuse me. During the wide open throttle run the target AFR is 12.75. During the run Lambda 2 is mind buggeringly well behaved, almost like it was painted on the screen at 12.7. If you are running open loop at wide open throttle and bank 2 is measuring 12.7 with a target of 12.75 you have essentially no O2 error. I would be incredibly happy if my open loop AFR remained so close to target. Or are you running closed loop? Lambda 1 is bouncing around a lot more during the wide open throttle run and is more like what I am used to seeing.

The Inj6LambdaFB values are moving around during the wide open throttle run which adds to my confusion for 2 reasons
- are you running open loop and Inj6LambdaFB is just the calculated correction; but, is not being applied? If so your AFR is so close to target I don't know why there is any calculated correction because Lambda2 AFR is pretty much on target
- are you running closed loop and Inj6LambdaFB is being applied to correct the AFR? If so, the Inj6LambdaFB value is moving around during the run. You need some error to generate a correction and normally I would expect to see some deviation in the AFR before the correction occurs. It may be a scaling problem; but, I don't see any errors in the AFR that would trigger that correction.