Bowtie316

iTrader: (1)

Okay, so I have a wierd problem that I can't explain. What I have is a 2000 Silverado PCM running a 4.8 4l60E swap. I bought a 2002 pcm for another swap and scanned it and "write entire" the operating system to the 2000 pcm. I did this because I heard that it had better false knock resistance. Okay so it flashed fine, runs and drives fine. I spent a couple weeks dialing in the VE table using fuel trims, then dialed in the MAF freq. table. Had my LTFT running 2-3 max, most of the cells were 0 or 1 on a day to day basis. Had this wierd start "sort of hang for a second and then it would catch" issue going on that I couldn't get to tune out. Also idle was a little bit wierd.

So fast forward a couple weeks later and I needed to borrow the battery out of this vehicle for a couple hours. Hook it back up and it's hard to start, had to give it a little bit of gas but it started. Then it hunts around for idle a bit and finally settles in. Okay so I assume the IAC reset and was finding zero. So after this, my idle and starting is fixed, but my LTFT are way off. Like anywhere from +16 to +22.

How could an IAC reset change the base fueling tables? Even if the air is bypassing the throttle body, it still doesn't bypass the MAF. I can't figure out why this happened, but I am working on correcting the tune now.

The only thing that I can think of is the 2000 and 2002 have different IAC park positions and it thought it was parked somewhere else.

I would assume that if I would have replaced the entire pcm instead of just the OS and Calibration that this wouldn't have happened. Might be a good idea to do the IAC relearn prior to tuning if you change OS.

Anyone ran into anything like this before?

jimmyblue

iTrader: (11)

Idle air alone, ought not to affect fuel trims. More
likely some "fellow traveler" or common cause.

MAF has a pretty low contribution to the blended
air mass used at lower throttle. Of course if the MAF
is big-time off at the low end, even its small weight
in the sum could matter. But MAF is not going to
change its behavior across a battery swap. The
bypass air amount changing, though, passes through
that weighting to the air mass number.

You might like to pull a log under conditions similar
to one you have saved, and compare details at the
places you're not liking things - is it the air mass
that's way different, or for same air mass has the
injector PW moved, is there qualitative difference in
the NBO2 waveforms, etc.

Bowtie316

iTrader: (1)

I agree that it is likely something other than IAC because I would think it would only affect idle and low throttle LTFTs, but this is across the board it seams.

I'm not sure I agree that the MAF has a low contribution at lower throttle. Reason I say this is one time I only tuned MAF freq using a LTFT+STFT vs MAF Frequency log and got my LTFT down to 0-2 across the board, idle on up. I completely left the VE table alone.

Good idea on looking at logs and seeing where the difference comes from, I'll do that and post back later.

Bowtie316

iTrader: (1)

I didn't find anything odd in the scan. LTFT were all 14-18 roughly across the board. I'll throw a new tune at it and see what it does, but it has me baffled.

thunderstruck507

iTrader: (18)

Ditto.

MontecarloDrag

iTrader: (3)

I think you didn't reset the fuel trims when you tuned the VE and MAF tables. You tuned the tables on top of whatever the trims were.

You may need to tune it again, reset trims every time you make any change to the tune

Bowtie316

iTrader: (1)

I reset trims and confirm the trims reset everytime I flash a new calibration. I'm kindof **** about it. Not to mention I had driven this for 2 weeks with that tune and monitored on several different occasions that they were all 0-2 LTFT. Actually on a particularly humid day they were going negative by 1 or 2 indicating less oxygen available which I would expect.

Is there anything else that resetting the ECM will reset that you can't reset with the scanner?