Stumped by intermittent Misfire

[91nsx85crx]

When the engine is cold it starts fine and runs fine & wideband reads 14.7 at steady state and high 12's under acceleration. After it is up to operating temp if I shut the car off & restart it, it starts fine but then while driving it misses and will not accelerate over say 30mph. The O2 reads very lean & erratic around 17-20/1 but the fuel pressure is a stable 50-55lbs. It does not throw any error codes and after several minutes of driving may suddenly start running normal again. I'm thinking it might be a thermally related ignitor issue or perhaps crank angle sensor. But I think a faulty or intermittent crank angle sensor would throw an error code. Any suggestions before I purchase an ignitor to try out? It definitely seems ignition related. For reference it has new coils, good plugs, new walbro 255l pump. It is a 91 and a newly rebuilt balanced & blueprinted ready for turbo build. I'm just running it off the stock ecu normally aspirated to break it in. Everything is seated in well now and it is running great other than this gremlin.
Thanks, Todd

 

[pgilliam1]

Usually:

Ignitor problem-
Get CEL misfire code
Misfire under accelleration
OK when cruising
Intermittant problem

Throttle Positioning Sensor-
Hesitation
Lack of power (especially in 4th, 5th and 6th gears)
No CEL
Higher than normal idle (1200 RPM)

I have no experience with the crank angle sensor. Try this thread;
http://nsxprime.com/forum/showthread.php?t=52292&highlight=crank+sensor

Good Luck!

 

[Larry Bastanza]

No misfire codes to read, it is a 91

I sounds like the car is OK when it is running "OpenLoop" fuel trim. Which means it is not using the 02 sensors to adjust fuel mixture. Once warm, it will use the 02 sensors and this seems to be when you are running poorly.

As an experiment, disconnect one of your 02 sensors and get it to throw a code(CEL on) This will keep the ECU in "OpenLoop" mode all the time. See if the problem disappears. If so the ECU is not trimming correctly which could be based on many things, like compression ratio, pump volume, and so on.

Regards,
LarryB

 

[91nsx85crx]

Larry, thanks for the suggestion. I'll try it for "shits & giggles",however, it will run fine when fully warmed up, half an hour of driving, it is not until it is shut off and sits a little bit and there seems to be some heat soak to the heads & intake.(you'd burn your hand touching the ignitor)then when you restart it, it is missing. When I drive it like that after several minutes of driving, it will all of a sudden start running fine and I can continue to drive it and it will run fine.
Any thoughts? Thanks.

 

[ftuhy]

One possible cause is a bad or loosely seated connector. Metals (and plastics) expand and contract with temperature, and as a result if the connector is not perfect, you can get a bad connection at some temperatures. The same thing happens with bad integrated circuits (ignitor, ecu, etc) -loose connections fail at some temperatures but work fine at other temperatures.

Good luck with this ugly problem. Maybe my comments will help.

 

[Stevon]

95 NSX-T Stumped by intermittent Misfire

I too have an ultra lean problem and no power under 3K rpm after warm up and closed loop conditions. See what my issues are under thread: http://nsxprime.com/forum/showthread.php?p=1479687&posted=1#post1479687 My car is an early 95 ODB2 car without J1962 diagnostic connector. I wired one in using SAE pinout and finaly can log real time data. The problem is that I do not have the experience to interpret the pages of data to extract a viable solution. Car was running rich and now seems to run lean. Have spent hundreds of dollars and replaced many, many parts but the car can not be registered or safely driven. Went to a great deal of trouble to get the car to run and stay in closed loop but that has not fixed the drivability or emissions issues. Any ideas? such a complex problem..........

No misfire codes to read, it is a 91

I sounds like the car is OK when it is running "OpenLoop" fuel trim. Which means it is not using the 02 sensors to adjust fuel mixture. Once warm, it will use the 02 sensors and this seems to be when you are running poorly.

As an experiment, disconnect one of your 02 sensors and get it to throw a code(CEL on) This will keep the ECU in "OpenLoop" mode all the time. See if the problem disappears. If so the ECU is not trimming correctly which could be based on many things, like compression ratio, pump volume, and so on.

Regards,
LarryB

 

[pgilliam1]

Re: 95 NSX-T Stumped by intermittent Misfire

I too have an ultra lean problem and no power under 3K rpm after warm up and closed loop conditions. See what my issues are under thread: http://nsxprime.com/forum/showthread.php?p=1479687&posted=1#post1479687 My car is an early 95 ODB2 car without J1962 diagnostic connector. I wired one in using SAE pinout and finaly can log real time data. The problem is that I do not have the experience to interpret the pages of data to extract a viable solution. Car was running rich and now seems to run lean. Have spent hundreds of dollars and replaced many, many parts but the car can not be registered or safely driven. Went to a great deal of trouble to get the car to run and stay in closed loop but that has not fixed the drivability or emissions issues. Any ideas? such a complex problem..........

Did you replace the fuel pump resistor?

 

[Larry Bastanza]

After some additional thinking and experiences over the last week, give a REALLY good look to all of the grounding connections in the engine bay.

Regards,
LarryB

 

[d1 guy]

Does it do this at all the time after it warms up or does it only happen when you drive for awhile, park, turn it off, then go back to it later and drive it? The reason I ask is because on one of my other NSXs, I had a head gasket issue. At cold start in the mornings, it would be fine. If I drove it for awhile and parked it, coolant would leak down on my pistons and cause an intermittent mis-fire. I noticed this mostly after I drove it to work in the mornings and let it sit until lunch. Thats when the issue popped up. After it burned off, it would drive fine. It would do this for 15 minutes or so and then would be fine. After Barney diagnosed it and replaced the head gasket, the issue never showed back up. You can look at the top of the cylinders and see if there is any coolant on them. Just drive it for awhile and then let it sit for a couple of hours. Mine was showing up on cylinder 3 I think.

 

[Valhalla]

I too have heard of the head gasket issue. If it's not injectors (you've replaced about everything else!) I might look at the gaskets too.