temporary loss of power during acceleration :(

[goldNSX]

When you go on the throttle, the vacuum hose on the FPR raises the fuel pressure. As your pump is (was) weak, it could not supply the required amount of fuel to raise the fuel pressure. That's why you notice a difference. The most difference can be felt in much more torque below 4k rpm. At least my car was a dog compared to another car with a good fuel pump.

PM sent.

 

[enkrypt3d]

......

 

[jwmelvin]

It's the heatsink-looking thing just above the fuel filter. Replace it with a wire to test your fuel pump. It's also possible to jump it on the cabin side of the firewall where the relay that engages it is, but easiest in the engine bay.

 

[enkrypt3d]

Thanks and bypassing it permanently isn't a bad thing is it? I have the walbro high flo fuel pump as well.

 

[darock]

I ran the complete season this year with the bypassed resistor. No issues from a driving point of view.

BUT i had issues passing the yearly emissions tests. The engine is running too rich at idle with the resistor bypassed even with a weak fuel pump. That's also what LarryB on prime says.

It's only minor and I do not see any differences in fuel consumption so it's nothing to be really concerned about.

I will swap the fuel pump next year when the engine comes out for lma and clutch replacement.

Bernhard

 

[enkrypt3d]

I ran the complete season this year with the bypassed resistor. No issues from a driving point of view.

BUT i had issues passing the yearly emissions tests. The engine is running too rich at idle with the resistor bypassed even with a weak fuel pump. That's also what LarryB on prime says.

It's only minor and I do not see any differences in fuel consumption so it's nothing to be really concerned about.

I will swap the fuel pump next year when the engine comes out for lma and clutch replacement.

Bernhard

Very interesting..... so if bypassing the resistor "fixes" the issue then we need a new resistor? Or with the high pressure fuel pump, the resistor is just getting in the way?

 

[Jinks]

I ran the complete season this year with the bypassed resistor. No issues from a driving point of view.

BUT i had issues passing the yearly emissions tests. The engine is running too rich at idle with the resistor bypassed even with a weak fuel pump. That's also what LarryB on prime says.

It's only minor and I do not see any differences in fuel consumption so it's nothing to be really concerned about.

I will swap the fuel pump next year when the engine comes out for lma and clutch replacement.

Bernhard

I thought the whole process of bypassing the resistor tells you that the resistor itself is bad not the fuel pump? Given the motor acts right with resistor bypassed?

 

[enkrypt3d]

anyone have a part # for the fuel pump resistor?

 

[darock]

why should the resistor go bad? That's a solid part.

The pump surely wears after 200+tkm and 25 years.

An interesting temporary fix would be to find/build a replacement resistor with half or 2/3 of the original resistance.

Bernhard

 

[goldNSX]

I thought the whole process of bypassing the resistor tells you that the resistor itself is bad not the fuel pump? Given the motor acts right with resistor bypassed?

Obvious reasoning but not the case. The fuel pump is the only of the two items which has moving parts inside. Moving parts wear out over time. A resistor does not (if will blown up if it gets severly overheated). With a worn fuel pump an additional resistor lowers the voltage at the fuel pump and reduces its fuel delivery even more. In the case of the NSX to the extend that you get hesitation. The cure pill is to bypass the resistor so the fuel pump gets higher voltage all through the rev band (12V instead of 9V below 4200 rpm). If you ask yourself why such a small voltage drop results in this the answer is that (esp. Denso) fuel pumps react very significant to voltage variations as fuel pump tests have delivered: http://www.stealth316.com/2-fuelpumpguide.htm

This is a band aid as your fuel pump is still weak and on a track day with heavy fuel demands could let you run lean. But as darock observed it takes quite a long time until you get there. In the long run you should replace the fuel pump. That's a task I've the extraordinary pleasure to do today.

 

[enkrypt3d]

Obvious reasoning but not the case. The fuel pump is the only of the two items which has moving parts inside. Moving parts wear out over time. A resistor does not (if will blown up if it gets severly overheated). With a worn fuel pump an additional resistor lowers the voltage at the fuel pump and reduces its fuel delivery even more. In the case of the NSX to the extend that you get hesitation. The cure pill is to bypass the resistor so the fuel pump gets higher voltage all through the rev band (12V instead of 9V below 4200 rpm). If you ask yourself why such a small voltage drop results in this the answer is that (esp. Denso) fuel pumps react very significant to voltage variations as fuel pump tests have delivered: http://www.stealth316.com/2-fuelpumpguide.htm

This is a band aid as your fuel pump is still weak and on a track day with heavy fuel demands could let you run lean. But as darock observed it takes quite a long time until you get there. In the long run you should replace the fuel pump. That's a task I've the extraordinary pleasure to do today.

That's strange as I replaced my fuel pump about 3 years ago and it appears the hesitation issue has come back. I tried removing the philips head screw on the resistor last night but it's nearly stripped... ugh

 

[darock]

I haven't found it in the service manual. Does anyone know the resistance of the resistor box? It is absolutely possible that the resistor box suffers from vibration but most likely it will either fail completely or have extremely high resistance making the car undriveable.

Other thing to check is, where the voltage for the fuel pump comes from. I have seen in the service manual that the main relay is on the way of the current. Bad solder joints and/or worn relais inside the main relay can easily alter the voltage to the fuel pump. Especially with the main relay known for causing issues due to bad solder joints. Easy fix though.

Bernhard

 

[dablackgoku1234]

Not sure if Walbro pumps have the same longevity as OEM. But since you have a walbro, you must have a fuel pressure regulator/gauge. Just make sure it can handle the extra flow of the Walbro when jumped at 12v.

 

[darock]

I have too much fuel pressure at idle with the weak stock fuel pump running at full speed as it runs rich at idle. I have seen that while doing the emissions test. So the regulator is too small.

No idea how the stock fuel pressure regulator would manage the extra flow of the stronger walbro pump without the resistor. Have you ever checked the fuel pressure?

Bernhard

 

[goldNSX]

That's strange as I replaced my fuel pump about 3 years ago and it appears the hesitation issue has come back. I tried removing the philips head screw on the resistor last night but it's nearly stripped... ugh

Actually, I don't rule out any other components which might went bad. As you mention to have changed the fuel pump I think you can rule that out. Larry Bastanza has a thread about hesitation which points to the O2 sensors.

When you put in a Walbro I would consider an AEM FPR or the like, not the stock one. The later tends to get overflown by a fuel pump with more flow.

BTW I hate the Walbro fuel pump, it's cheap (a lot of copies on the web too...), in price and in quality. I just come back from my Toyota Supra fuel pump install. Super quiet, I don't hear it at all but it obviously likes to be feed with heavy current: 16 A instead of 6.5 A of the noisy, humming, overall bloody Walbro 255 LPH. I'm glad I've thrown it out and that I'm back to a JDM quality part again. Never ever Walbro again. It's not worth the hassle IMHO.

 

[darock]

O2 sensor related hesitation feels different. Its more like misfire, like messing up with the clutch on acceleration, when it feels like riding a bull. Also only happens after cruising oder while cruising because the 02 sensors need temperature to work.

The fuel pressure hesitation is more like the engine has not much power or a delay in power delivery when you instantly press the throttle. That effect instantly goes away at 4000rpm.

Bernhard

 

[Old Guy]

I have too much fuel pressure at idle with the weak stock fuel pump running at full speed as it runs rich at idle. I have seen that while doing the emissions test. So the regulator is too small.

No idea how the stock fuel pressure regulator would manage the extra flow of the stronger walbro pump without the resistor. Have you ever checked the fuel pressure?

Bernhard

Something to consider.

Fuel pressure regulators can be very sensitive to the back pressure on the fuel return line. Normally the FPR tracks manifold vacuum and adjusts the fuel rail pressure to maintain a constant pressure differential between the fuel rail and the intake manifold. Increased back pressure on the return line can alter the regulator's operation such that it is not able to drop the rail pressure low enough to maintain that constant differential between the rail and the manifold. At that point the fuel being delivered through the injectors increases and the engine may start running rich (if the increased fuel delivery is beyond the O2 trim limits - which may or may not generate an O2 out of limits error code).

At idle, the flow in the return line is highest with the highest back pressure on the regulator's return line. Also, at idle the intake manifold pressure is the lowest with the result that the FPR is trying to drop the rail pressure to its lowest value to maintain a constant pressure across the injector. If the FPR is sensitive to back pressure, this can result in the engine developing a run rich at low engine load condition which may disappear at higher engine loads.

If the OEM FPR is sensitive to back pressure, by-passing the resistor may increase the back pressure (even with an ailing pump) to the point that the idle AFRs start to drop below target. Installing an aftermarket high flow fuel pump will also probably cause the same problem. Bigger is not necessarily better. The effect of the larger pump should be easy to check using the FPR test procedure in the shop manual. If your idle fuel pressures are nicely in the pressure range set out in the manual, you should be good to go. If they are at or above the upper end, you may have created a problem for yourself.

As a related note, installing an aftermarket FPR may not solve the problem. On another project car I installed an Aeromotive adjustable FPR which was advertised as being able to operate with base pressures down to 20 psi. I did this in an attempt to improve idle operation by reducing the injector offset and increas pulse widths with a lower fuel pressure on some rather large injectors. I discovered that at idle the FPR could not drop the rail pressure low enough to maintain the base pressure across the injectors causing the ECU to generate lots of O2 correction. After some follow up with Aeromotive's technical support line the conclusion was that the back pressure on my rather long return line was too high. Their recommendation was that 3/8" was a minimum and that given the length of my return line bigger would, in this case, be even better.

 

[darock]

The fuel lines on the NSX are fairly short and standard size. Don't see why this would cause issues.

Bernhard

 

[Old Guy]

It may or may not. Only testing will tell and doing the fuel pressure check as per the service manual is a simple procedure and will confirm whether there is a problem. If Larry B. (post #30 ) says that by-passing the fuel pump resistor drops the AFR, then something is causing the fuel pressure to go up.

 

[goldNSX]

The AEM FPR offers you three orifices with different Diameter for the return line to choose from. I had made my mind which to install but was lucky to install the right one. So there must be some evidence in this.

 

[darock]

The AEM FPR offers you three orifices with different Diameter for the return line to choose from. I had made my mind which to install but was lucky to install the right one. So there must be some evidence in this.

Yes, especially in your application with the stronger than stock fuel pump. The stock fuel system is definitely not ready for flow rates way higher than it was designed for.

Maybe I will buy a pressure gauge and do a test. Hook up the resistor again and see what pressure I get at idle and what happens when I bypass it again.

Eventually I do that also before and after I resolder the main relay.

What I still don't get about all this is the fact that Honda designed the "slow stage" of the pump so near the failure point. Just a little drop in efficiency of the pump causes the issues with it being plenty strong enough to power the engine at full tilt and even overpower the fuel system at lower revs.

Bernhard

 

[goldNSX]

What I still don't get about all this is the fact that Honda designed the "slow stage" of the pump so near the failure point. Just a little drop in efficiency of the pump causes the issues with it being plenty strong enough to power the engine at full tilt and even overpower the fuel system at lower revs.

Bernhard

I would not call it 'so near the failure point'. It simply just shows 'there' first. The car has to tell you somehow it needs a new part (ok, it took 3 years in my case to understand what it needs). It's better than running lean at WOT and 8k rpm. Generally the 'slow stage' is there to keep noise down. In the lower half of the rev band there's no need to have so much fuel which is returned to the tank.

Years ago when this forum was much more technically there was a discussion about the limits of the fuel system. Esp. the feeding lines and connections at the fuel filter and the return lines.

 

[enkrypt3d]

I would not call it 'so near the failure point'. It simply just shows 'there' first. The car has to tell you somehow it needs a new part (ok, it took 3 years in my case to understand what it needs). It's better than running lean at WOT and 8k rpm. Generally the 'slow stage' is there to keep noise down. In the lower half of the rev band there's no need to have so much fuel which is returned to the tank.

Years ago when this forum was much more technically there was a discussion about the limits of the fuel system. Esp. the feeding lines and connections at the fuel filter and the return lines.

so what new part do I need? I'm sure the fuel pump is fine....

 

[goldNSX]

so what new part do I need? I'm sure the fuel pump is fine....

Before throwing parts at the car what tests did you accomplish already?

Did you bypass the fuel resistor?
Did you unplug the O2-wiring and testdrive the car?
As you have a Walbro 255 LPH did you also install an upgraded electric wiring? Did you measure the voltage at the fuel pump?
How old is the fuel filter?

Things to check before you go the expensive route. If you can't perform them yourself get the car to a trustworthy mechanic.

 

[Dr. JL]

Install new fuel injectors. I had the same symptoms with new fuel pump/regulator/resistor and used (but cleaned) fuel injectors. New injectors fixed the hesitation. Expensive, but worth it.