I recently put the stock LS1 back in my 1999 trans am (selling it), and had the computer flashed by horist to a stock 2002 pcm flash, w/ EGR, AIR, and the rear 02 sensors deleted. The car has stock intake, manifolds, cats, 28# injectors, & stock MAF sensor. Ever since the day I started it, it has not run right. At first it wouldn't idle correctly. I replaced the passenger side 02 sensor, and tightened a ground strap, then it idled fine. Now that being fixed is where the weird problems start.

90% of the time I drive the car, it hesitates and spudders when you first press the pedal down. Usually if you hold the pedal down alittle when it's hesitating, it will fix itself and continue on, and other times it will act like it is misfiring badly. On the occasion it is running decently, when you cruise at the same speed for more then a few seconds the SES light starts blinking, but the car throws no codes, and has 0 misfires in the history.

I recently was able to scan the car, and this is what I found....

On idle (which is fine), the front 02 sensors swing between 10-900mv's, as I believe they should ... and the rears stay steady around 450mv's (I guess that's how he deleted them?). When you step on the gas, the front 02 sensors drop to exactly 13mv's, and stay pretty much right there until you let up on the gas.

The LT and ST fuel trims are also very weird. The LTFT seems to be stuck around +20% at idle, and the STFT is around +15% at idle. When you give it gas, both of these drop towards 0%, never going negative. MAF = 3211hz, and reads 10-16 g/s ... and the MAP reads 0.8-1.0 mv's. If I'm reading this info correctly, it looks like when you step on the gas, the O2's tell the car it's running lean, and the PCM then pulls fuel for some reason, making it even leaner, and causing the car to spudder. I have no clue why it would do this, and more importantly how to fix it. Again, the car does not throw any codes at all.

Any suggestions?

 

Any chance the misfire codes were 'tuned out' of the PCM? From my understanding a blinking SES is an emissions related troublecode, most likely misfire.

 

Is it a stock '99 motor with '99 cam, '99 heads, '99 maf, etc.? If so, an '02 tune won't work. You need a '99 tune with !AIR, !EGR, and the codes tuned out. The only tables you should have from an '02 are the injector tables.

 

Really? Horist told me they are identical, which was why he would just pop in an 02 tune, since the only thing that was different would be the injector table. So you think I need to have the computer reflashed with a 99 tune, then swap over the injector table, !EGR, !AIR, etc?

P.S. If I unplug the MAF and IAT sensors and put the car into limp mode, it runs pretty much perfect .... if that helps.

 

The VE table, spark tables, fuel trim settings and quite a few other tables are different between 99 and 02. I don't have a comprehensive list since my compare feature doesn't work between operating systems. But, I do know there are big enough differences to cause the problems you're experiencing. The biggest difference is probably in the timing. I gave my friend (00 TA) my timing table after his cam install to be safe & free of KR. His car ran, but way underpowered because the 02 timing tables have anywhere from 4 to 14 degrees LESS timing at WOT. Put the all of the original '99 settings back in, kill the codes, and swap injector tables. Worst case scenario - I'm wrong and you still have the current tune saved on your laptop. But, I have a good feeling I'm not wrong on this one. Either that, or it's not in the tune...it's mechanical.

 

Well, unfortunately I dont have LS1edit or any of those tuning things, so I would need to get a shop or someone on the board who was nice enough to toss a stock flash back in for me.

I think you could be right, since I've checked and double checked pretty much every sensor now. I'm 99% positive it's not mechanical, since when you put it in limp mode the car runs awesome. Seems to definately be a tuning/computer related issue. I have a feeling you're probably right on this one.

Any idea who I could get to reflash this for me? I had Horist do it the first time, but he hasn't answered any of my PM's since I got the PCM back from him for some reason.

 

The O2's drop to 13mv?? Thats one problem right there. I WOULD say you might have a clogged cat(s) as well, but you said it runs fine when its in limp mode.

 

Yup. When I had the 402 in, I had an air/fuel gauge that measured the 02's voltage ... that thing used to jump around like crazy during everything but WOT driving. Now, it seems to jump from one extreme (10-20mv) to the other (800-900mv), rarely hitting anything in between.

I had headers, so I purchased the manifolds/cats/y-pipe from one of the guys here on the board who had taken them off a car with ~2,000 miles to put headers on, and I did a quick visual inspection in which everything looked brand new.

And yes, the car runs pretty much perfect in limp mode.

 

looking at the tune now... it was a 99 OS that was put back into the PCM (the 99+ hardware is the same, not the Operating systems or calibration details). However there looks to be a problem w/the IFR table, the offset table is correct, but it looks like the IFR table is off

If you want send me the PCM again and I'll fix the IFR table and send back at my cost

 

I had the same problem and comes to find out that I had two popped sparked plug wires, so give them a try

 

You found the problem? That's awesome, this has been buggin me forever.

I'll send you the PCM and paypal you the money to send it back, it's not fair for you to pay for it. PM me with your address and ur paypal ID.

Thanks mike.

 

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Horist, I've sent you a few PMs but didn't get anything back... Not sure if my PM's are screwed up or something. Do me a favor and toss me a PM or post as to whats up with the PCM.

Thanks.