'91 NSX Rough Engine up to 3500 RPM then smooth at higher RPM

[T38Flyer]

Anyone have an idea of what would cause my 91 NSX (39000 miles) to run rough, slight miss, up to 3500 RPM, but at higher RPM runs completely smooth?

All coil packs and spark plugs were replaced a year ago, and it ran fine. Rough running just recently began. Happens when both cold and normal operating temp.

Never tracked, just a monthly driver. Fantastic car overall.

 

[Old Guy]

I can't remember the exact conditions when the ECU by-passes the fuel pump resistor - 3500 RPM seems a little low. If your fuel pump is getting tired it may not be providing sufficient fuel pressure when the fuel pump resistor is switched in at low engine loads. Under heavy engine load the ECU by-passes the fuel pump resistor causing the fuel pump to run faster and increase delivery volume and pressure. This may be restoring the correct fuel pressure which results in smoother running.

The test to confirm this would be to measure the fuel pressure at idle as set out in the service manual. If the pressure is low, you may have a failing fuel pump. However, you could also have a dirty fuel filter (when was it last changed) or you could have a bad electrical connection at the fuel pump resistor or the resistor is failing (test procedure and test values are in the manual). The first step is to measure the fuel pressure to determine whether the problem is related to fuel pressure. If the pressure is low, the next step is to determine whether it is the pump, fuel filter, fuel pump resistor or something else.

 

[SWFL_NSX]

Misfires are always tough, maybe especially on obd1. More information would be useful-

When was the last time the following been replaced/serviced/cleaned?

-Fuel Strainer
-O2 sensors
-Fuel injectors and seals
-Throttle Body
[MENTION=7701]Oldguy[/MENTION] is absolutely correct about the fuel resistor cutting out between 3500 and 4k and fuel pressure check would be a good diagnostic if the above are up to date on their service. Pulling plugs may also help isolate a particular cylinder.

 

[T38Flyer]

Thanks so much for the thorough reply! Will definitely look into possible fuel pump related issues.

 

[T38Flyer]

Thank you!

 

[asylum]

Bypass the fuel resistor. If car runs fine, then something is wrong with the pump (though fuel resistor could also be out of spec or fuel filter cloged, but that is rare).
O2 sensors could also be the problem. Disconnect them. It will throw a CEL. But if car runs fine, then something is wrong with at least one of the two sensors.
If problem persists, come back here.

 

[goldNSX]

I can't remember the exact conditions when the ECU by-passes the fuel pump resistor - 3500 RPM seems a little low.

4200 rpm.
The mileage is little low for a bad fuel pump unless the car has seen a lot of time on the track where the plastic impellers within the fuel pump wear out faster.
But both tests (unplugging the o2-sensors and shorting the fuel resistor) are very easy tests to perform, just perform one after the other.