STFT goes crazy at random times

[neond123]

So. I'm beginning to tune my car, I drive it to get coffee everyday, it now has an official 12 miles on it.

But I've disabled the VE tables so I can adjust the MAF. Unfortunately I can't really take the RPM's above 3,000 and I can't really get out of second gear. (I'm not on a main road)

Either way, I've got the LTFT/STFT err table for the MAF and I've got it dialed in pretty good. Seems to run well, drive well, not nearly as much off-idle hesitation as there was. But every once in a while, usually after I reset fuel trim to get another sample run, the short term (and sometimes long term) fuel trim will go APE. Like 25% both banks, both widebands pegged 10.5:1, and it almost drowns the engine out, you can feel it almost stalling.

What causes this erratic fuel suffocation?

Thanks in advance.

[blindsquirrel]

EVAP purge? Hard to say without knowing what components are in use in this 'my car' you mention.

[B52bombardier1]

Vacuum leak? I recently had the same situation due to two other things:

1 - Overly oiled K&N air filter. This allowed an oil film to confuse the tiny heated wires in the MAF. Cleaning these wires solved part of my LTFT problem.

2 - I had my air filter attached to a plastic four inch elbow and a length of PVC pipe. Apparently, this did not provide enough "straight air" into the MAF - air was too chaotic and confused the MAF. Once I attached the K&N filter straight onto the MAF, my LTFT dropped to random +/- 10% values - good.

The funny oddball thing was my +25 added fuel LTFT values never set a trouble code or check engine light. Still haven't figured that out and the only way I found my bigly LTFT values was by watching the streaming PIDS. This is on a 1999 GMC Sierra 5.3 liter LM7 donor into a 1970 Chevy El Camino.

Rick

[neond123]

Vacuum leak? I recently had the same situation due to two other things:

1 - Overly oiled K&N air filter. This allowed an oil film to confuse the tiny heated wires in the MAF. Cleaning these wires solved part of my LTFT problem.

2 - I had my air filter attached to a plastic four inch elbow and a length of PVC pipe. Apparently, this did not provide enough "straight air" into the MAF - air was too chaotic and confused the MAF. Once I attached the K&N filter straight onto the MAF, my LTFT dropped to random +/- 10% values - good.

The funny oddball thing was my +25 added fuel LTFT values never set a trouble code or check engine light. Still haven't figured that out and the only way I found my bigly LTFT values was by watching the streaming PIDS. This is on a 1999 GMC Sierra 5.3 liter LM7 donor into a 1970 Chevy El Camino.

Rick

No :/ doesn't sound like a problem I would have. It's a Holley 90mm with a reducer to a brand new MAF with a Spectre isle 5 special AutoZone filter right after.
No oil, no vacuum leaks as far as I can TELL.
MAF err values/LTFT+STFT are usually right in the +/-4 range with the occasional outlier. Good enough for me to move on to VE. But I hesitate to do that if it's going to keep going apeshit.

Thanks for any input

[S10issues]

I'm having a similar problem at idle then it clears up. Any chance it could be COT? My thought was maybe the computer is throwing extra fuel to save cats based off of some math I don't understand. Just a thought.

[neond123]

I'm having a similar problem at idle then it clears up. Any chance it could be COT? My thought was maybe the computer is throwing extra fuel to save cats based off of some math I don't understand. Just a thought.

Unfortunately I've had my COT disabled from the jump.
Decided to just disable closed loop and give up on fuel trim for now and just tune using wideband error.