BigE

iTrader: (2)

I have a car that is getting serious negative LTFTs. I can clear the fuel trims and it immediately starts heading way negative, ie. -10. This is a stock vehicle. Nothing has been changed including the air filter. If everything is working properly, is the O2 sensor the only parameter that would change the FTs?

John_D.

iTrader: (2)

Yes, the o2 sensor is what drives the pcm to start making corrections.

If it's fouled it can't see the oxygen in the exhaust and will tell the pcm that you're running rich. Then the pcm will start pulling fuel to "fix" the problem...

radkon

iTrader: (2)

Look at the air filter and intake track. Make sure there are no obstructions. I had a mouse nest in one of my cars airbox several years ago which led to serious airflow issues.

BigE

iTrader: (2)

John D., that's what I was thinking. I went into the PCM controls and turned the Fuel Trim Learn off. I can see where it stopped updating the LTFTs. It doesn't appear that the STFTs were updating either. If I had a fouled O2, would turning off the fuel trim learn eliminate the problem (for testing purposes)? I tried it and the car didn't knock like it had, but it still bogged and was seriously down on power.

Thanks

John_D.

iTrader: (2)

Just saw your question(s) today.

Turning off the learn should hold the trims steady for testing.

Getting rid of a lean knock sounds like it did something.

If it had been picking up knock before, the pcm probably switched over to the low octane timing table. It will take some time to drift back to the high octane table, so depending on how much lower the low octane table is set, that may be your bogging/down-on-power situation right there.

BigE

iTrader: (2)

Turns out that it was a leaky injector. Injector cleaner added to the tank didn't fix it, but a professional injector cleaning did. Car is back to it's normal self. Thanks, John D.