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Flat below 4500 rpm

N

NONSENSE

New Member
Joined
19 September 2009
Messages
8
Location
Cary, NC
I've had the car for a little more than a year, and this problem has been occurring for a couple of months. Performance is really flat below 4500 rpm, then the power comes on like it should (full throttle, as they say!). It's like driving a turbo. Problem "feels" fuel related.

I checked out the forums, and I jumpered out the fuel pump resistor. The power was there at all rpm, so I figured it was the fuel pump resistor. I bought a new fuel pump resistor, but when I installed it the problem was still there. I replaced the fuel filter (external, not the one on the fuel pump), same problem. I've also changed out the air filter, for what it's worth.

Car is a 1992, manual transmission, with 107,000 miles.

There are still some things I could try, the Service Manual says to physically measure the fuel flow. I should probably also check the resistance of the new fuel pump resistor, but I'm assuming it's okay since the problem is the same on the old resistor and the new one.

Does anyone have any ideas or suggestions? My understanding is that the fuel pump resistor causes the fuel pump to put out a lower volume of fuel below ~4500 rpm, then the pump gets full voltage and pumps at maximum volume. This seems to be what is happening - at rpms below 4500 the fuel flow just isn't high enough unless the resistor is jumpered out. Is there something else in the circuit below 4500 rpm that could be causing this fault? Can I drive the car long-term with the fuel pump resistor jumpered out (it's sooo much more fun to drive it when it develops full power)?

Thanks,
Daniel Exum
 
I would check the new resistor with an ohmmeter to make sure it isn't bad.
There should be a spec in the manual for what the resistance should be.

The resistor is what drops the voltage to the fuel pump. There is also a relay that will switch circuits to bypass the resistor at higher rpm's so that the fuel pump gets fulll voltage(ie. more fuel pressure) if I am not mistaken.
 
It would be easier to understand a "tired" fuel pump if the thing didn't take off like a banshee at ~4500 and run strong to redline. Also, acceleration is strong when the resistor is jumpered out.

The strong performance above 4500 also makes it hard to believe that the problem is ignition related. I have changed plugs, but no difference. The plugs and filters needed doing anyway, but it doesn't make sense to change the fuel pump with such good performance above 4500. I suppose it's possible that the fuel filter on the pump inlet (in the tank) is partially clogged, and the flow is low at reduced voltage. When the pump goes to full voltage the partial clogging wouldn't matter as much. Is this likely?

The CEL isn't lit, btw.

Thanks,
Daniel
 
I replaced the two O2 sensors about six months ago. The CEL was on and the codes indicated the front sensor, as I remember, but I changed both. As the forums say, what a PITA! Of course, replacing them doesn't guarantee that they are still working correctly, so I will pull the connectors and see what happens (that's easy enough).

I'll also see if any codes are stored - I haven't checked that lately.

But why would jumpering out the fuel pump resistor cause the problem to essentially go away?

Thanks,
Daniel
 
TitaniumVtec said:
Unplug both of your O2 sensors and take it for a ride after the CEL comes on. If the power is restored, your O2 sensors need to be replaced. This is LarryB's advice and worth its weight in gold!
Click to expand...

Yes, but NONSENSE already sorted it out by bypassing the fuel pump resistor. So, the O2-sensors are not the problem here. By high chance your fuel pump 'cries' for a replacement. You an measure the resistance but resistors don't go bad so easily.
 
I did check the O2 sensors by unplugging them and test driving. Although it did seem to be a bit better (maybe richer mixture in fuel map when these faults are detected?), there was still a definite difference when rpms climbed to 4500. The difference is dramatic, like hitting boost with a turbo.

I re-connected the O2 sensors, then cleared the codes by removing the clock fuse. I also jumpered out the fuel pump relay at this time, and went for another drive.

No doubt about it, full power is restored when the fuel pump resistor is jumpered out. I cannot tell a difference in power as the rpms move through 4500, everything is fine.

I then checked the resistance of both the new resistor and the old one that I had removed, both checked at 0.6-0.8 ohms, depending on how I held the ohmmeter leads. I'd say that they are the same, the question is whether they are both good? I looked in the Service Manual, and I can't find the specification for the fuel pump resistor, does anyone know what it should be?

At this point, unless I'm missing something it seems that there are only three things that it could be:
fuel pump regulator,
fuel pump relay,
fuel pump itself.

The relay seems unlikely, as the pump does work at low and high rpm. The pump also seems somewhat unlikely, as there's plenty of fuel flow at high rpms with resistor in place (although it may be possible that the in-tank filter is clogged and just will not flow adequate fuel at lower voltage).

Can anyone offer any help? Has anyone had trouble with their regulator? The Service Manual specifies how to check the regulator, is that my next step?

Thanks,
Daniel
 
Test the fuel pressure. Make sure it is within specs.

This sounds very similiar to my problem. I went through tons of different parts and it turned out to be a weak fuel pump.

The biggest indicator for me, was that the fuel pressure was lower than specs.

Once I replaced the fuel pump, the car worked like it should. I had a more linear power throughout the rpm band, and no more surge at 4500 RPM.

I used a Walbro 255lph fuel pump instead of the OEM. It was much cheaper than the oem fuel pump and it was a good prep. if I went FI.
 
It's either the resistor or the pump. My bet is it's the pump. You can put a fuel pressure guage on these external fuel filters really easy (eBay for about $25 with the banjo bolt). Fuel pressure at idle should be about 40psi. Good Luck and let us know. Happy Motoring!
 
Last edited:
Well it was the fuel pump. Big D had an extra sock unit with ~20,000 miles on it that he gave me gratis, and it worked wonders. Car has linear power from 1,000 through to redline. Wow, it feels GOOD!

Thanks to all for their help. Thanks especially to Big D for the pump.

Daniel
 
It seems like the biggest indication of a "tired" fuel pump is the hesitation around 4000K then a strong surge.

Or you can flow the direction in the service manual and test the fuel pressure.
 
You know, I think I notice just about the same issue with mine 4k-4.5k but it's not that obvious.

Is there a downside to running full psi at all times?
 
Last edited:
NONSENSE said:
Well it was the fuel pump.
Click to expand...
BINGO! The fuel pump dies slowly. The ECU regulates 'back' if not enough fuel is delivered. The engine still runs smooth but with less torque below the point the fuel pump is forced into stage two (4200 rpm) and the fuel pump resistor is bypassed. If the deliverd fuel volume is very low the ECU can't handle it anymore and you get hesitation. Shorting the FPR is the test.
 
I just purchased my first NSX Prime on the internet. When it arrived. The first drive was very disappointing. It has taken about 3 weeks to find this thread.
Reason being, I had a similar issue. I didn't notice that 4500 rpm was when the acceleration cleaned up as noted here. Here is what I noticed:
At low rpm say 2500 to 3000, push the throttle, and really no response at all, then it would slightly kick and begin to accelerate, but not as expected.
The only thing I did notice, was that the VTEC did engage at the proper place (4800)
I experienced essentially no power through the entire RPM range.
When I put a "high tech" paper clip into the plug that I disconnected from the Fuel Pump Resistor, the acceleration is now normal and great!

I am really happy to find this thread!!
And I LOVE my Prime!!!

- - - Updated - - -

nsx-13.jpg
 
JacksonPrime said:
I just purchased my first NSX Prime on the internet. When it arrived. The first drive was very disappointing. It has taken about 3 weeks to find this thread.
Reason being, I had a similar issue. I didn't notice that 4500 rpm was when the acceleration cleaned up as noted here. Here is what I noticed:
At low rpm say 2500 to 3000, push the throttle, and really no response at all, then it would slightly kick and begin to accelerate, but not as expected.
The only thing I did notice, was that the VTEC did engage at the proper place (4800)
I experienced essentially no power through the entire RPM range.
When I put a "high tech" paper clip into the plug that I disconnected from the Fuel Pump Resistor, the acceleration is now normal and great!

I am really happy to find this thread!!
And I LOVE my Prime!!!

- - - Updated - - -
Click to expand...

Shorting the fuel pump resistor is not a fix its just a trouble shooting technique. I would not run the car long term like that since there is a problem that needs to be fixed. Weak fuel pump, bad or disconnected fuel pressure regulator or weak o2 sensors.
 
Last edited:

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