Rev and idle issues after a long rest

[Super_Billoz]

good morning everyone, i have a problem that i can't figure out. i have a 1992 nsx that unfortunately has been left standing for several years, when it was restored the following were changed:
timing belt,
water pump,
clutch,
service belt,
spark plugs,
cleaned tank,
o2 sensor,
alternator,
head gasket,
disassembled and cleaned and completely restored the intake manifold,
replaced the air sensor,
replaced the fuel pressure regulator with an aem,
replaced oil and filters.

now the car starts perfectly, when cold it holds the idle but when it is hot the idle is rough, it goes up and down continuously, the eacv system was cleaned and replaced gaskets and the small filter.
in addition to that the car runs badly, every now and then, pressing the accelerator at 4000 rpm it floods, every now and then the vtec doesn't engage. yesterday a test was done by disconnecting the tps and the pump resistance and the car was running perfectly, today I put everything back and it runs very badly, only disconnected the tps and it still doesn't work. the only thing that comes to mind is the fuel pump, could it be?

thanks to everyone and sorry for my english

Cheers from italy
Nicola

 

[asylum]

Car runs ok with the fuel pump resistor bypassed ?

Check your fuel pressure. Could be fuel pump but also just the resistor or the fuel filter.

 

[Old Guy]

Have you used the ECUs service check connector to determine if there are any other stored error codes that would provide some information about what the problem might be? If the ECU has been reset the engine will need to be run until it comes up to operating temperature and then driven a bit before any errors are detected.

I agree with @asylum that checking the fuel pump resistor would be the easy first place to start. In particular, you want to measure the actual resistance to make sure it is within specification and you want to check the electrical connections for the resistor and the wiring as the heat from the resistor can cause these to deteriorate.

If the fuel pump is original it is now 42 years old and depending on the number of kms it may be getting worn. As noted by Asylum, by-passing the resistor speeds up the fuel pump motor which can compensate for a worn pump mechanism; however, if that is the case you should plan for pump replacement.

I assume that when you said "replaced oil and filters", filters included the fuel filter. If not, I would do that. Then I would carry out the fuel pressure test and the fuel pressure regulator test as described in the service manual. Passing the fuel pressure test in the service manual is a basic test because it only measures fuel pressure at idle. It does not guarantee that you can maintain pressure under high engine output. There is also a a fuel flow test (page 11-102) which would provide indication whether the fuel pump is failing. Inspect all the electrical connections from the main EFI relay (the main EFI is the relay that controls the fuel pump) right to the pump terminals looking for bad connections that may cause low voltage to the pump. Your list did not include a new main EFI relay which is a common problem point for the NSX. If the main EFI relay is original and has not been removed and had its solder connections re flowed then I would install a new one (lots of posts discussing main relay problems and repair).

You did not mention fuel injectors. If the car was in storage for several years with gasoline in the fuel rail I would take the injectors out and have them cleaned or install new injectors. This will require new seals for the injectors when the clean or new injectors are put back in.

I am not particularly fond of aftermarket fuel pressure regulators. I don't know about AEM; but, have had poor experience with Aeromotive and some other regulators. The specifications for the acceptable fuel pressure test values are for the factory pressure regulator so you will have to be the judge as to whether your AEM regulator meets the specifications in the service manual.

 

[Super_Billoz]

Hi guys and thanks for the replies. Tomorrow I will do another test by disconnecting only the resistor. The pump and relays are still the originals, I have a walbro 255 ready to mount. In the meantime I will buy a new relay. Yes, the fuel filter has been replaced and the injectors have been overhauled and cleaned. Tomorrow I will do more tests and let you know. Thanks

 

[Old Guy]

This is the fuel pump circuit


If you just disconnect the fuel pump resistor the car will not start because the fuel pump is not getting any power. You need to by-pass the resistor with a jumper connecting a blk/yel to the blk/red wire. You can do this right at the resistor connector plug (some people use a paper clip as a jumper) or at the plug on the fuel pump relay

 

[Spoolinfool]

Make sure you have a fuel pressure gauge set pressure to factory specifications.

 

[Honcho]

Hi Nicola-

I would not install the Walbro pump and would replace the AEM regulator with the Honda Genuine part (16740-PR7-A00). It's much more durable than the AEM and will easily last 20+ years. The AEM unit is really only needed if you have a supercharger or turbo and require higher fuel line pressure. We recently had another member who experienced problems with his AEM after only about 1 year. (@R13)

Also, members here have had nothing but problems with the Walbro pumps and they tend to fail or cause issues after only a few years. I would replace it with the OEM Denso unit (17040-SL0-930). The Denso lasts for 200,000km and 30 years. You'll never have to touch it again.

While I agree that fuel pump/pressure issues could be causing your 4,000 rpm problem, something you said in your original post is a clue that you may also have an idle problem. You mentioned that the engine holds idle steady when it's cold, but will hunt and surge when it's warm. The high/low switching of the fuel pump is not controlled by coolant temp, but a table in the ECU that tells the pump to change between high and low speed based on several factors like RPM, throttle position, etc. Thus, if it were a fuel pump problem, the car would always run rough regardless of engine temp.

I think you may be experiencing a throttle body idle passage restriction. When the engine is cold, the FITV feeds extra bleed air to the intake manifold to hold a steady cold idle speed. As the FITV closes while the engine warms up, it eventually shuts completely at normal engine temp. In a warm engine, idle bleed air is being delivered to the manifold by the IACV (controlled by the ECU) and the base idle bleed screw in the throttle body. The weird part is that the IACV control logic is "dumb" in that it expects a certain amount of bleed air from the screw in order to work properly. If your bleed screw passage is clogged, it doesn't deliver enough air for the IACV to work right and the idle is all over the place. You can check this function by warming the car up and unplugging the IACV. If the engine dies or the idle drops to 300 rpm and surges, the idle passage is clogged with soot.

Remove the throttle body and remove the idle set screw (it's painted in place so it will be hard to unscrew). If you look inside the throttle bore, you'll see a small hole near where the butterfly plate touches the bore on the top of the bore. This is the idle air passage and it exits in the idle screw recess. Shoot carb cleaner through this hole in both directions until the fluid runs clear. Re-install everything and follow the base idle setting procedure in the service manual. The car should idle steady at 600-700 rpm with just the idle screw open and the IACV unplugged.

 

[Old Guy]

On a stock engine, I share @Honcho 's enthusiasm for the OEM fuel pressure regulator and fuel pump. You used to be able to purchase just the bare Denso pump for around $100 (I have a spare sitting in a box in my garage); however, you can't do that anymore so you have to fork out 4x$ for the complete fuel pump assembly. I would still do that versus an aftermarket pump.

Honcho's observations about the idle control circuit are correct. However, I respectfully deviate from his diagnosis because in the original post you said

yesterday a test was done by disconnecting the tps and the pump resistance and the car was running perfectly,

I assume you meant shorting out the resistor because if you disconnect the pump resistor without a jumper in place the engine will stop (or has the wiring on the car been modified?). If it idles correctly with the pump resistor by passed that seems to me like an air/ fuel mixture problem. I suggest check the fuel pressure first. If the fuel pressure tests out OK then follow Honcho's advice on checking the idle control circuit.

 

[R13]

Everyone above knows what they're doing, but yeah my short list would be:

• Put a fuel pressure gauge on the fuel filter so you can see what it's doing. A basic analog gauge won't be pricey and could immediately point to a solution.
• If you're motor is stock, I'd second/third going with the factory pressure regulator. I'm running an adjustable one by necessity, but you'll have to give me 10 years of no further issue before I recommend an AEM (and even then I'd suggest cleaning/rebuilding it upon arrival new).
• Coils would be the other avenue I might explore alongside the fuel issues. The symptoms sound like something I experienced with my neglected-at-the-time '92 just after I bought it. "Common wisdom" was that coils rarely go bad, so probably not the issue, BUT they're susceptible to moisture shenanigans and mine all ohmed correctly cold, but a couple of them would arc or change resistance when they got hot and cause issues but it was inconsistent enough not to throw codes every time.

 

[Super_Billoz]

hello everyone, sorry but i only had time today to try the nsx.
i haven't been able to test the iacv yet but i installed the fuel pressure gauge.

as i said i mounted an aem regulator (i bought the aem regulator and the walbro pump because they are easier to find in italy, if they give me problems i will change them again). the standard aem regulator was set very low, 25psi, so the car with the resistance disconnected (jumper) was fine, i proceeded to set the pressure as per the manual, (about 40psi) and the car started to get worse, it couldn't go above 7600 rpm and it ran badly.

the idle problems have remained the same.

besides testing the iacv, could the problems be caused by the pump?

Thanks a lot guys

 

[Old Guy]

"about 40 psi" is below the specification in the service manual if you tested using the procedure in the service manual (engine running vacuum reference hose disconnected). Your fuel pressure regulator should be adjusted to give you a fuel pressure of a minimum of 46 psi (engine running vacuum reference hose disconnected). The correct base pressure would be between 46 psi and 53 psi with no by-pass on the fuel pump resistor. If you can't get the base pressure into the correct range then there is a problem with the fuel pressure regulator or the fuel pump or the power supply to the pump or any combination of these causes.

If you want to diagnose by replacement then you can replace the fuel pump and see if that achieves the correct base pressure and if that does not work then replace the fuel pressure regulator to see if that achieves the correct base pressure. However, diagnose by replacement means that you really do not know what the problem is. Whatever you do I would not fiddle with the IACV or attempt to adjust the base idle speed until you get your fuel pressure into the correct range.

The NSX ECU has a fuel cut-off rev limit at 8300 RPM. If you are basing your 7600 RPM rev limit on the dashboard tach, there is a high probability that the accuracy of the tach has drifted due to ageing of capacitors in the tach circuit. The dash tach is an analog measurement and is separate from the digital speed measurement that the ECU calculates. It would be normal for the accuracy of the tach to have drifted on a 32 year old car. On the later OBDII equipped cars you can read the engine RPM calculated by the ECU to check the dash gauge. The only way for you to check your dash gauge is to use an external tachometer with an inductive pick up on the ignition service check loop.

In my opinion, I would not be checking high RPM / high engine load performance until I get the base fuel pressure set up correctly. Have you used the service check connector to determine if there are any stored error codes in the ECU?