Stumble and Missing at low RPM when Hot (Intermittent)

[xmacdaddy]

Hello all, Wanted to throw out a problem that has seriously been pissing me off. For about the last 3-4 months about 1 out of 4 trips out in the NSX, once the engine is hot it slowly starts to run more and more rough, especially at low RPM and under load (going up hill, etc.). It seems like one or more cylinders is not firing. twice it has gotten so bad that I could barley get it to rev up after a stop light. When it was real bad I did hear 1 or two little back fires. Seems to me like a spark issue, maybe a bad coil pack or two? Are there are connectors that are common culprits? Oh, and I did just replace the plugs a few weeks ago with not effect.

thanks so much for any tips, would love to take care of this DIY.

1991 w/ 80k miles

 

[kookoo4nsx]

Could be 10 different things but the simplest is to make sure your battery cables are tight and corrosion free. From there I would check:

1) Pull all coils and check resistance. I had 2 that passed resistance but still were missing.
2) Do you store your car outside? Moisture can arc the coils.
3) Swap a good known main relay and igniter.

 

[xmacdaddy]

Thanks man, yes the car is outside. But actually, my battery contacts have been a bit suspect. couple months ago I the car was dead and I just tapped down on the battery clamps/contacts. I'll give those a good clean and figure out a way to tighten better.

Thanks for the other tips, Good idea about checking resistance of coils, didn't realize there was a spec for that.

 

[pgilliam1]

Very intermittant and only when hot -hmmmmm- sounds electrical to me.

Check all grounds (engine, battery, ignitor).

Check the ECU for stored fault codes first then go from there.

 

[Larry Bastanza]

mmm..... One out of four trips. Does the stumbling coordinate with a low fuel tank?

Regards,
LarryB

 

[xmacdaddy]

I do not believe it has anything to do with low fuel tank. I will keep a eye on that though. Since I heard a few back fires, that makes me think that it is getting fuel but not igniting. My understanding is that a back fire is when fuel get ignited inside the exhaust system, is that correct?
'
For give my ignorance, but what is the "Igniter". Does that control the coil packs?


thanks again

 

[pgilliam1]

I do not believe it has anything to do with low fuel tank. I will keep a eye on that though. Since I heard a few back fires, that makes me think that it is getting fuel but not igniting. My understanding is that a back fire is when fuel get ignited inside the exhaust system, is that correct?
'
For give my ignorance, but what is the "Igniter". Does that control the coil packs?


thanks again

The ignitor controls the spark to the coils.
Backfires usually happen from improper timing of ignition when the exhaust valve is open.

 

[ATERPAK]

Pretty much the same problem I just started having except it's all the time. I've swapped the ignitor and it wasn't the problem. I do have a new main relay and I'm thinking of changing it out as a simple check. Battery is fine and so are the connections and voltage. I'm checking fuel pressure and the regulator next. Car seems to run fine with revs higher than 4K RPM and when there is little to no load on the car. Put a load while in a higher gear while on the freeway or even from a dead start and it stumbles and hesitates until it reaches 4K in RPM's and then goes smooth. Also during a stop at idle I get minor backfires.

I'll let you know what I find as I'm going through all the possible things it might be.

Al

 

[pgilliam1]

Pretty much the same problem I just started having except it's all the time. I've swapped the ignitor and it wasn't the problem. I do have a new main relay and I'm thinking of changing it out as a simple check. Battery is fine and so are the connections and voltage. I'm checking fuel pressure and the regulator next. Car seems to run fine with revs higher than 4K RPM and when there is little to no load on the car. Put a load while in a higher gear while on the freeway or even from a dead start and it stumbles and hesitates until it reaches 4K in RPM's and then goes smooth. Also during a stop at idle I get minor backfires.

I'll let you know what I find as I'm going through all the possible things it might be.

Al

That sounds exactly like a bad fuel pump resistor. Short the connector to keep the pump at full pressure. Easy test.

 

[redacuransx921]

hello i have had the same prob a few times in the past it would sound like a subaru when driving it would back fire at idle and it only did it when i gave the car a bath or drove it in the rain. Water drains right on the rear bank right on the coil packs i have seen water in there as well i have replace the rubber gasket on the coil pack cover and thought that would work to keep the water out it did but some still gets in there i have also notice one time of messing with the mess i took the three coil packs out and and blew them with a air gun around the seams and water came out so i would think water moisture is getting up there and the coils packs are not as tight as they should be. maybe seal them up with a bead of silicone but not sure if you can do that they maybe need to release heat from the split? so now i dont drive in the rain and put a plastic trash bag over the rear bank when i wash make sure motor is not hot and take it off before you start it :biggrin:

 

[Whatwasthat]

Pretty much the same problem I just started having except it's all the time. I've swapped the ignitor and it wasn't the problem. I do have a new main relay and I'm thinking of changing it out as a simple check. Battery is fine and so are the connections and voltage. I'm checking fuel pressure and the regulator next. Car seems to run fine with revs higher than 4K RPM and when there is little to no load on the car. Put a load while in a higher gear while on the freeway or even from a dead start and it stumbles and hesitates until it reaches 4K in RPM's and then goes smooth. Also during a stop at idle I get minor backfires.

I'll let you know what I find as I'm going through all the possible things it might be.

Al

Same exact symptoms I am having here. I just changed the plugs since I havent driven the car in a few months. Ohm tested the coils when they were out and they all passed between 0.9 ~ 1.0 ohm. I do suspect an O2 sensor being the culprit when I'm in closed loop, as the car runs smooth at first start up, then starts missing after warm up. Last year I did get a CEL, when I pulled the code if I remember correctly, it was for a rear O2 sensor. When I get a chance I will unplug the O2 sensor and see what happens.

My car is a 91 with autorotor CTSC.

 

[ATERPAK]

Well it seemed that the problem/s were this:
1. Put in a new set of plugs.
2. Pulled the injectors and had them cleaned and flow tested back to like new and consistent from one cylinder to another.

From there the car seemed to be running fine. I took Larry's advice and had the fuel pump changed but that didn't help nor hurt the situation. The car seems to be running fine now and my next big test will be passing smog in California. Hydrocarbons are close for this model year and hopefully I can find a smog station who will work with me and make sure the car is hot when the test is done and the traction control is turned off.

Al

 

[Whatwasthat]

Problem solved.

1. Replace spark plugs - problem was still there when warmed
2. Replace faulty O2 sensor - Test drive vehicle for over an hour and no more misfire or hesitation.

My car being a 91, O2 sensor was due to replace. I should replace both of them, but since I had such a difficult time removing the one that was suppose to be very easy, I decided to wait on the front O2 until I just replace the manifolds with headers. So now, anybody have any headers or 97+ manifolds for sale?