Low Engine Power before 4k RPM

[jsullyboy]

This was my first summer with my 91 NSX. It had 36.5k on it when I purchased it. I did about 500 miles thus far.

I have done a number of repairs over the summer. A lot of deferred maintenance and replaced items that have failed or were failing due to age.

The latest project is to address the low power at low RPMs.

The car seems to lack torque and power in low RPMs until it gets to about 4,000 RPM. Also it does not fire up right away. It seems to crank longer than I would expect it to need to start. (not extreme, but my S2000 fires up super fast) Since I never had the car from new and have never driven one other than my own I have no frame of reference. It could be that this is how it normally is but, it does not seem right to me.

Reading other posts on this subject I see that the usual suspects are:

- Fuel Filter
- Fuel resistor
- Fuel pump
- O2 Sensors

I looked through the service records I got with the car and I see no evidence that that fuel filter or fuel pump have ever been changed.

The first thing I did after purchasing the car was to change the water pump, timing belt, spark plugs. [I have done many other things mostly unrelated to the engine. Clutch master, slave, ABS flush and unstick solenoids, gauge cluster repair (almost caught on fire), AC restoration and interior restoration like windows, broken trim etc.]

My plan is to start with some injector cleaner first. Also I will order a fuel filter. I don't see anywhere in the records it has ever been done.

In other threads I see references to by-passing the fuel pump resistor. I could use some advice on this. I assume that by-passing means putting a jumper on the following connector?




Doing this means that the fuel pump will run full power at all times? Then if I notice a boost in power. It could be the fuel pump or the resistor itself? The fuel pump does not have a lot of milage on it. But its old and has probably sat for long periods in between use.

The oxygen sensors

If they were failing wouldn't the check engine light come on? Or could they be failing but just not enough to trigger the code? Reading though other threads I have seen a debate on if they would cause this issue or not. In some threads I have seen replacing them fixing a similarly described problem.

The suggested test I see for this is unplugging them and running without them. By doing this the engine would default to a base program. I assume unplugging them will trigger the check engine light. I have a few questions on this:

- What is the easiest way to reach them to unplug them? Under the car or can it be done from the top?
- Is putting a jumper on the service connector and counting the flashes on the dash the only way to read the codes from the ECU?

If I needed to replace these sensors how hard are they to remove? I have seen examples where they needed to be heated up with a torch and a sensor socket to remove them. Is this the norm? My car has never been driven in the elements. The bolts connecting the muffler are very rusted though. I can't see the manifolds. I had no problem getting the sensors off my S2000. But they are in a super easy to access location. Seems like replacing the back O2 requires removing the muffler to access it.

Fuel pump

I ordered the fuel pressure gauge from SOS. I will follow the procedure to test in the service manual.

Any advice or suggestions would be much appreciated!

 

[Old Guy]

The NSX normally starts quickly for a car of its vintage. Modern cars can typically sense TDC on any cylinder and can start the ignition timing calculations almost immediately; but, the NSX may need close to a complete crank rotation before it can start the ignition timing sequence so it is not instantaneous start like my 2021 Audi. My 2000 will typically fire up with less than 2 seconds of cranking.

The biggest contributor to slow fire up is loss of fuel pressure in the fuel rail while parked. This means that the pump prime cycle is not long enough to fill and bring the fuel rail up to specified pressure. The easy way to check for this is let the prime cycle complete without engaging the starter, turn the ignition off and then let the prime cycle complete a second time (or third time) to make sure the rail is pressurized and then engage the starter motor. If this gives you a fast start then you have a leakage problem somewhere that is causing loss of pressure in the fuel rail during shut down. This could be:
- leaky no return valve in the fuel pump
- fuel pressure regulator allowing leakage out the return line
- fuel injector(s) that leak
1 you can live with. 2 you can live with providing your test confirms that the regulator operating pressure is not compromised. 3 you should probably do something about because its a symptom of injector problems.

I would not bother with add-in-the-tank injector cleaners. The cows will come home before they have a material effect. That is more of a continuous maintenance thing if you are not using top tier gas. If the injectors are leaking or questionable I would remove them and send them out to somebody like RCInjector services for cleaning.

That location in the photo is where you would install a jumper (paper clip) to short out the fuel pump resistor and yes it will cause the pump to run at full power. If that resolves your problem that does point a finger at a problem with the fuel system. However, that could be a fuel filter problem or a fuel pump problem.

Running the car with the O2 sensors should put the car into open loop fuel control and should set an error code immediately. The engine normally runs in open loop until it has warmed up so this will only make a difference on a hot engine. If the heater fails in the O2 sensor that will trigger an immediate O2 sensor error code. However, it is possible for the O2 sensors to deteriorate to the point where they are causing fuel mixture issues; but, have not crossed the threshold where they qualify as completely dead. If you know what you are looking for and have access to a high quality voltmeter with high input impedance you can determine whether the O2 sensors are faulty. The best tool for doing this is an oscilloscope because the voltage swings up and down and on cheaper digital meters with slow up date rates it just looks like the numbers are jumping around. The service manual shows the connector location which is as I recall up top; however, your 1991 is different than my 2000. I have heard from authoritative sources that on the early cars with the cast iron manifolds it is the front sensor that can be a bugger to remove.

Right now your problem is too loosely defined to conclusively say that you have an O2 sensor problem and in fact that would not be high up my probably cause list. I would start with the fuel system first. If the car did not have regular use for a long period of time then the injectors might be a really good place to start. Low mileage; but, with regular use would be less of an issue because at least the injectors are being operated regularly. With clean injectors you can get an accurate fuel pressure test which might indicate a pump, filter or FPR problem. If the fuel system checks out 100% then you can go sniffing around the O2 sensors.

 

[jsullyboy]

I put in fuel injector cleaner and was doing the test described here. Got the check engine light to come on.



Going to attempt to read the code(s).

 

[jsullyboy]

The NSX normally starts quickly for a car of its vintage. Modern cars can typically sense TDC on any cylinder and can start the ignition timing calculations almost immediately; but, the NSX may need close to a complete crank rotation before it can start the ignition timing sequence so it is not instantaneous start like my 2021 Audi. My 2000 will typically fire up with less than 2 seconds of cranking.

The biggest contributor to slow fire up is loss of fuel pressure in the fuel rail while parked. This means that the pump prime cycle is not long enough to fill and bring the fuel rail up to specified pressure. The easy way to check for this is let the prime cycle complete without engaging the starter, turn the ignition off and then let the prime cycle complete a second time (or third time) to make sure the rail is pressurized and then engage the starter motor. If this gives you a fast start then you have a leakage problem somewhere that is causing loss of pressure in the fuel rail during shut down. This could be:
- leaky no return valve in the fuel pump
- fuel pressure regulator allowing leakage out the return line
- fuel injector(s) that leak
1 you can live with. 2 you can live with providing your test confirms that the regulator operating pressure is not compromised. 3 you should probably do something about because its a symptom of injector problems.

I would not bother with add-in-the-tank injector cleaners. The cows will come home before they have a material effect. That is more of a continuous maintenance thing if you are not using top tier gas. If the injectors are leaking or questionable I would remove them and send them out to somebody like RCInjector services for cleaning.

That location in the photo is where you would install a jumper (paper clip) to short out the fuel pump resistor and yes it will cause the pump to run at full power. If that resolves your problem that does point a finger at a problem with the fuel system. However, that could be a fuel filter problem or a fuel pump problem.

Running the car with the O2 sensors should put the car into open loop fuel control and should set an error code immediately. The engine normally runs in open loop until it has warmed up so this will only make a difference on a hot engine. If the heater fails in the O2 sensor that will trigger an immediate O2 sensor error code. However, it is possible for the O2 sensors to deteriorate to the point where they are causing fuel mixture issues; but, have not crossed the threshold where they qualify as completely dead. If you know what you are looking for and have access to a high quality voltmeter with high input impedance you can determine whether the O2 sensors are faulty. The best tool for doing this is an oscilloscope because the voltage swings up and down and on cheaper digital meters with slow up date rates it just looks like the numbers are jumping around. The service manual shows the connector location which is as I recall up top; however, your 1991 is different than my 2000. I have heard from authoritative sources that on the early cars with the cast iron manifolds it is the front sensor that can be a bugger to remove.

Right now your problem is too loosely defined to conclusively say that you have an O2 sensor problem and in fact that would not be high up my probably cause list. I would start with the fuel system first. If the car did not have regular use for a long period of time then the injectors might be a really good place to start. Low mileage; but, with regular use would be less of an issue because at least the injectors are being operated regularly. With clean injectors you can get an accurate fuel pressure test which might indicate a pump, filter or FPR problem. If the fuel system checks out 100% then you can go sniffing around the O2 sensors.

Thank you for your reply "Old Guy". After putting in the injection cleaner I did a test drive. In 3rd gear from 2500rpm I floored it. Very slow response but the check engine light did come on. Once the car hits 4500rpm its like turbo kicks in there is such a difference.

The accelerator pedal increases resistance the further you press it. Is this normal? Sorry for these simple questions but I never drove the car factory new.



I jumped the check service connector. When fishing it out I noticed the grey connector that was not plugged into anything. Another newbie question. Is this normal? Looked around and could not see a place to plug it into.

With the service connector jumped I see 2 quick flashes. Which should be code 2 if I am interpreting this correctly.


View attachment Check Engine Code.mp4

According to the diagnostics it seems this is the rear oxygen sensor.



Please let me know if I am interpreting this correctly.

Thanks!

 

[TANTO]

I can only comment on the "simple" questions you asked. The pedal pressure on my car stays consistent throughout it's range. Since i have no rear O2 sensor on my 92 (header install) i would be very surprised if it's malfunction caused a big lapse in torque. I'm sure Old Guy and others will comment on your code (you got a check engine light before your test?). IMO you really need to find a ride in another NSX for a baseline. Regarding torque - in the 90's when i drove a Tercel to work the torque of the X felt pretty good, but daily drivers today have so much more torque. I equate our ride to a high end 600cc sport bike where for a short "sortie", you just always keep the R's up a bit to stay in the torque band. Similar to the Ferrari 348 of the time. My V8 torque monger friends who have driven my car are not impressed with the torque. Their loss of course as it's my contention that if i let them drive it for a month they wouldn't want to give it back! So perhaps a slight difference in driving style will keep that smile on your face! The engine is certainly built for it.