"Solved" my engine sputtering/hesitation problem

[mr_goat]

Just wanted to share my happiness after finding the reason for my engine sputtering problem. First, I was certain that my problem was related to bad connection between the coils and spark plugs due to corrosion but since the problem was prominent at less than 4500 rpm I thought i'd try the fuel pump resistor bypass procedure and TADA! Silky-smooth operation through the entire register :biggrin:

So a new fuel pump will be ordered, I'm going for the Walbro.

I suggest all people who are having engine sputtering problems to try this REALLY easy 5 second test, just to exclude the fuel pump being the problem or like me, finding that it was the problem.

Unplug the resistor (match-box sized thingy mounted just above the fuel filter) and use a paperclip and short-curcuit the connector. Go for a test drive.

 

[Jin1976]

I had similar problem and it was clogged injectors. Diagnosed by a Japanese HKS technician.

 

[NSX2398]

Unplug the resistor (match-box sized thingy mounted just above the fuel filter) and use a paperclip and short-curcuit the connector. Go for a test drive.

I'm confused .. if you bypass the resistor and it works, then doesn't this mean the resistor is bad not the fuel pump?

 

[Supercar]

I'm confused .. if you bypass the resistor and it works, then doesn't this mean the resistor is bad not the fuel pump?

+1

 

[Supercar]

I had similar problem and it was clogged injectors. Diagnosed by a Japanese HKS technician.

Jin, how do you diagnose that?

 

[pgilliam1]

Doesn't make a whole lot of sense. Maybe the connection at the resistor was funky? Maybe the problem will return? Seems you should have replaced the resistor first.

 

[mr_goat]

Maybe you are right guys, but I'm thinking like this:

< 4500 rpm (or something like that, don't know the exact rpm) fuel pump resistor feeds low voltage to the fuel pump

> 4500 rpm fuel pump resistor feeds high voltage (13.7 volts?) to the fuel pump

A good fuel pump would be able to keep the fuel pressure at a satisfying level in both cases, but a worn-out pump might only be able to do that in the second case and up as the low voltage in the first case would not be enough. Bypassing the resistor would restore the fuel pressure in the first case also, but since the fuel pump is slowly failing, the problem will arise again if it isn't replaced soon (with the risk of running the engine lean).

It could also be a faulty resistor, somehow feeding the fuel pump with lower voltage than it is supposed to in the first case, in which case the problem would dissappear when > 4500 rpm.

Either cases are possible for me, and will both change the fuel pump and throw away the resistor. Just in case. Will be replacing the fuel filter also when I'm at it (it's time according to the service schedule).

 

[NSX2398]

For the benefit of the community .. and since it sounds like the sputtering is not intermittant, can I ask that you replace the resistor first to see if that is the problem and report back? I presume that replacing the resistor is much easier than replacing the fuel pump anyway and you might find out that the fuel pump is fine and doesn't need to be replaced. But even if you plan on doing the pump too, it would be nice the final resolution to the Wiki.

 

[mr_goat]

For the benefit of the community .. and since it sounds like the sputtering is not intermittant, can I ask that you replace the resistor first to see if that is the problem and report back? I presume that replacing the resistor is much easier than replacing the fuel pump anyway and you might find out that the fuel pump is fine and doesn't need to be replaced. But even if you plan on doing the pump too, it would be nice the final resolution to the Wiki.

A new resistor is about 40-50 bucks, and since I'm not interested in using one in the first place with the new fuel pump, I don't feel like throwing out the money for it, sorry.

The problem was highly intermittant when I first started to notice it. It was just noticable with a very slight hesitation around 2-3 k rpm but only some days, others it was fine. Then it became worse and worse where the car was almost not drivable some times, and just recently it became permanent.

 

[Jin1976]

Jin, how do you diagnose that?

Sorry to hijack this thread. In my case it was hesitation at idle, misfire when acclerating. After replacing and checking electrical parts, fuel resistor within spec, ignitor replaced, all ignition coils replaced, main replay replaced still the problem persist. So I brought the car to what I have thought to be expensive repair place, the Sing Hiap Hin workshop and the chief tehcnician Mr Ohura (from HKS) tested the car. I told him the problem was the fuel pump because I have asked another workshop and the only thing left to check is the fuel pump and they have the machine to test it.

Mr Ohura explain if the fuel pump is faulty the car would not even start. So he first check why the car sometimes cannot start. Turn out the content of the catalytic convertors have come apart so I asked him to clear the passage making them into a straight pipe with oem look. :biggrin:

Then solving the hesistation. One of the iginition coil caught some water in it so we replaced the iginition coils plus a new gasket to cover the coils. Solved the hesistation problem.

The misfire turns out to be clogged injectors. Mine was not service after 20 years and because of that some injectors were clogged. In order for the car not to run lean the computer pumps in more fuel causing it to run rich and at 4k rpm the car misfires. So he send the injectors to be serviced and check they are all within spec after 20 years.

All the service plus replacement cost about USD400. Now the car starts everytime I turn the key, no hesistation and no misfire plus extra loud HKS exhaust. I love driving the car in the basement parking lot. Even at low speed the rumbling caused the car alarms to go off.

Here is the pic of Mr Ohura. Yes his fav pant is pink. :tongue:

 

[Larry Bastanza]

Time for a fuel pressure gauge, and monitor the pressure with/without the resistor. Jumping the resistor out, will make the car run richer. But is fuel pressure really the issue? A little more digging is in order.. JMO

Check against specs and see what you get. Also make sure you do the vacuum regulator test as well.

Regards,
LarryB

 

[mr_goat]

Time for a fuel pressure gauge, and monitor the pressure with/without the resistor. Jumping the resistor out, will make the car run richer. But is fuel pressure really the issue? A little more digging is in order.. JMO

Check against specs and see what you get. Also make sure you do the vacuum regulator test as well.

Regards,
LarryB

I can see where you are going ^_^, based on your previous posts you recommend most people having hesitation issues to check their O2-sensors. But I agree, I would like to check the fuel pressure with and without the resistor and compare to specs. Will do in the nexst week when my vacations starts.

However, I don't believe you when you say the engine would run richer. First, you got the fuel pressure regulator, regulating the fuel pressure for the injectors. Secondly, the O2-sensors would detect the lower AFR (if they work) and the ECU would adjust the injectors accordingly. Or am I all wrong here?

And also, can you please elaborate on the vacuum regulator test?

 

[Briank]

Fuel pumps are very low failure items. Jumping out the resistor does not prove a bad fuel pump it just says the car runs better with more fuel pressure or in other words richer mixture. Lots of problems are masked by more fuel.

 

[Russ]

Fuel pumps are very low failure items. Jumping out the resistor does not prove a bad fuel pump it just says the car runs better with more fuel pressure or in other words richer mixture. Lots of problems are masked by more fuel.

+1 and I would add that resistors are also very low failure items.

 

[mr_goat]

Fuel pumps are very low failure items. Jumping out the resistor does not prove a bad fuel pump it just says the car runs better with more fuel pressure or in other words richer mixture. Lots of problems are masked by more fuel.

I'm listening to all you guys, I will have a closer look when I have time. Gonna do what Larry says first, check the fuel pressure. If the fuel pressure is good with the resistor plugged in, then it's not the fuel pump. I'll get back on that when i've done it.

Again, the fuel pressure is not regulated by the fuel pump, it is regulated by the fuel pressure regulator. Otherwise, changing the fuel pump and/or adding more fuel pumps would bump the fuel pressure enormously, the walbro for example flows 65 % more fuel than the standard pump. The mixture will not be affected by jumping the resistor or changing to a higher capacity fuel pump.

From SOS about the walbro: "This fuel pump works with both factory fuel pressures as well as applications with greater than factory fuel pressures... This kit is compatible with the factory pressure regulator, so it may be used on all factory cars with out modification."

Please explain to me, if I've misunderstood the mechanism with fuel pressure, how it works.

EDIT: Did some searching on the forum and it appears that the general percieved use of the fuel pump resistor is to reduce the sound from the pump for a more silent ride, since the fuel pump sits right behind the driver.

 

[NSX2398]

A new resistor is about 40-50 bucks, and since I'm not interested in using one in the first place with the new fuel pump, I don't feel like throwing out the money for it, sorry.

Sorry .. I misunderstood .. I thought it was your plan to replace both. But please keep us informed as to your progress and the final resolution.

 

[JacksonPrime]

Awesome! Awesome! Awesome. My acceleration was completely flat. I unplugged the Fuel Pump Resistor. Put a paper clip into the plug that was plugged into the resistor.
All is well! New fuel pump on order! Not looking forward to removing the tank though...

 

[davidf]

Awesome! Awesome! Awesome. My acceleration was completely flat. I unplugged the Fuel Pump Resistor. Put a paper clip into the plug that was plugged into the resistor.
All is well! New fuel pump on order! Not looking forward to removing the tank though...

Glad to see an old thread help out a new member. I too replaced my fuel pump and resistor a year ago. I did not test the resistor but replaced it as a cheap insurance. Hesitation was eliminated.

 

[Larry Bastanza]

I can see where you are going ^_^, based on your previous posts you recommend most people having hesitation issues to check their O2-sensors. But I agree, I would like to check the fuel pressure with and without the resistor and compare to specs. Will do in the nexst week when my vacations starts.

However, I don't believe you when you say the engine would run richer. First, you got the fuel pressure regulator, regulating the fuel pressure for the injectors. Secondly, the O2-sensors would detect the lower AFR (if they work) and the ECU would adjust the injectors accordingly. Or am I all wrong here?

And also, can you please elaborate on the vacuum regulator test?

Sorry for my late reply, forgot about this.

If the fuel pump/system is having delivery issues, clogged, worn, etc, meaning lack of fuel, the pressure in the system will be lower then required. If this is the case, it will run leaner, and stumble. So if you jump out the resistor, the fuel pump will run harder, delivering more fuel, as in richer. Now of course if you do this the ECU will see richer mixtures and will change the fuel amounts accordingly. So it is really about fuel volume, or lack there of.

This same issue can happen if the fuel filter is clogged, then perhaps the resistor jumping out may not work as well. Back to my original recommendation, check the fuel pressure.

I have also seen this throw a "1" of "2" code.

Regards,
LarryB

 

[skafia]

My car has the exact same symptoms, at low RPMs it feels like the engine is choking out (with no check engine light or anything). The interesting thing is that when I let my car sit for a few days to a week, then drive it, I have these issues and after about ~10-15 miles of driving it all goes away. I can reproduce this every single time. BUT, if I drive the car 2 days in a row, the problem does NOT happen on the 2nd day, any thoughts?

 

[Dr. JL]

Larry,
Why would a higher fuel pressure make an engine run rich? The ECU in closed loop mode will regulate the amount of fuel delivered to each cylinder via the fuel injectors for optimum performance, not the fuel pump.

 

[Lude_2_NSX]

I had the same issue, hesitation below 4K RPM. I installed a fuel pressure gauge (i would recommend everyone do this, its less than $50), and noticed I had only 30 PSI or so. Once I bypassed the resistor with a jumper wire, my fuel pressure jumped up to around 40 PSI (still low). At this point I knew the problem was the fuel pump getting weak.

I replaced the fuel pump with a Walbro 255 from SOS and remove the jumper, and my pressure was at a steady 45 PSI (perfect). No more hesitation.

I also replaced my fuel filter, and had my injectors professionally cleaned so I know the fuel system is 100%. Hope this helps future NSX owners out.

 

[darock]

same here.

Hesitation below 4000rpm. Was told to try bypassing the resistor. Hesitation gone and WAY better torque output below 4000rpm. 160k miles on the fuel pump. New fuel pump ready to be installed.
Having the injectors cleaned is definitely a good idea for higher mileage cars like mine. Planning to do that as well.

Bernhard

 

[Dr. JL]

I had similar problem and it was clogged injectors. Diagnosed by a Japanese HKS technician.

I had the same issue. Fixed with new injectors/spark plugs.

 

[06_typeS]

Great info on this thread. Does anyone have a photo as where the location of this resister is so I can try to bypass it tonight.

Thanks