Robert Foster

I have a 94 z28 lt1 m6. I've recently developed a misfire so I replaced the opti plugs and wire. That cured it for 2 days then it was back so I checked the plugs and all 8 ngks were fouled. So i figured it was 02s so I unplugged them got some new plugs got them in. Fired it up and took a drive had a little misfire so I headed home and parked it checked the plugs and they're starting to foul. Does anyone know what it can be

Injuneer

Is the engine stock or modified? What heat range are the plugs? Have you scanned the PCM for codes? Have you checked the fuel pressure? Have you checked for a leaking fuel pressure regulator dumping raw fuel into the intake manifold via the vacuum compensation line? Does the exhaust smell rich? Have you done any work recently that might have damaged the coolant temp sensor in the water pump housing? If the PCM thinks the engine is cold, it richens the A/F ratio.

When you disconnected the O2 sensors, did you also clear the learned Long Term Fuel Trims (LTFT) from the PCM? If not, any LTFT errors, due to faulty O2 sensors causing a rich A/F ratio are still stored in the PCM. The LTFT's (Cells 16, 17, and 18] will still be used to fuel the engine, even though the PCM will remain in open loop, due to the disconnected O2 sensors. It it would be unlikely for both O2 sensors to suddenly fail at the same time. Would be possible if you used leaded fuel, or used non-sensor safe materials using silicone for a repair.

Robert Foster

I don't have a way to scan yet. Fuel pressure is normal all the way around. How do I clear the long term fuel trims

Robert Foster

The engine is stock. The plugs are ngk tr55 copper plugs from auto zone. I checked the regulator and it's also fine. This is the second set in a row I've fouled. I just changed the opti and coolant sensor in the pump so I'm really confused on why its acting up

Injuneer

To clear the LTFT's, pull the “PCM BAT” fuse in the panel at the end of the dash for 30-seconds.

You're saying fuel pressure is normal - holds 40 PSI when pump primes, with engine off; within GM spec of 41-47 PSI at idle with vacuum compensation line off; drops about 8 PSI when you reattach the vacuum line? And there was no fuel in the FPR vacuum line?

Best approach, lacking a scanner, would be to download (free) Scan9495 software, get a cable, and run a data log of a cold start through warmup, including transition from open loop to closed loop. Then drive it normally for 10 minutes, with a varying combination of RPM and throttle. The log will show any codes, track all the sensor readings, and all PCM parameters. I can review the data log for you. Export a .csv file and upload it in a post in this thread.

https://www.camaroz28.com/forums/com...95-lt1-874306/

ROCKN SS

Pretty sure pulling the O2 sensors is going to put it in limp mode which will run rich, but shouldn't be bad enough to foul so quickly.

Along with resetting the LTFT, you might try cleaning the MAF sensor.

Injuneer

Limp mode is the process used to run the engine, as result of loss of almost all PCM input, or the ability to process that input. You'll know if it's in limp mode, because it will feel like the engine is barely running.

Losing the O2 sensors doesn’t put it in limp mode. It simply prevents the PCM from switching from open loop (no adjustments to A/F ratio calculations based on O2 sensor feedback) to closed loop (fuel trim adjustment based on O2 feedback enabled). Even with O2 sensors, every time you cold start your engine, the PCM ignores the O2 sensors for ~206-seconds (and until coolant temp meets minimum threshold, and O2 sensors hot enough to show activity). If you disconnect the O2 sensors, your engine will run exactly the same as it did after cold start. The PCM will be using the “learned” LTFT's to fuel the engine. The STFT's will not be enabled.

ROCKN SS

Yeah, I used the wrong term. But it is running richer, is it not?

Injuneer

Not necessarily. If the LTFT's were stabilized, even if they were wrong due to faulty input, it's going to run the same as it was with the O2 sensors connected. If the fueling is so far off that the PCM has to use BOTH the LTFT's and the STFT's to correct it in closed loop, it could run richer or leaner when you force it to remain in open loop, depending on whether the STFT’s were adding or subtracting fuel. Then there's the complication of open loop fueling being limited to Cells 16, 17, and 18. 17 and 18 are mathematical derivatives of cells 1 through 15 that are used in closed loop. Again, could run slightly richer or slightly leaner under various combinations of MAP and RPM.

If you're referring to the enriched cold start A/F ratio, in open loop the PCM looks at the CLT sensor, and leans things out as the engine warms up, eventually reaching the standard 14.7:1 target value. That's the reason for the timer being used for the transition to closed loop, and CLT temp requiring a minimum value before the transition.