My car is a '99 Trans Am M6, stock except for an exhaust cutout.

I had it on the dyno for the first time on Saturday, and it was off the charts rich. A TechII scan revealed that it's at about 9:1 A/F. O2s and MAF both seem to be fine.

What could be causing the car to be so rich on the stock tune? I'm afraid to tune it much leaner in case something else is causing it.

 

If the car is stock except for all but the cutouts it most likely through off your fuel trims. So in addition to the car running plain rich from the factory tune, because the FT's have seen positive values now as a safety measure the PCM adds a certain % of fuel to your WOT or PE table. This will cause you to see the rich AFR's that you are seeing. Time for a tune.

Dougie

 

Positive LTFT are lean condition...

 

So I can safely tune fuel out of it? I just have this strange feeling that someday the problem is going to correct itself, and then I'll be at about 30.1

 

what were the o2s reading? the car will run lean with the cut out, adding + fuel trims but it wont need those + fuel trims at WOT so it will make it rich. + fuel trims WOT for me were 975mV, i want to be around 900, the only thing i had to change to get back down was the fuel trims

 

When I scanned it a few days ago, they were running in the 980 range if I remember correctly. I had it tuned a little, and brought it down into the 920s

FWIW, here's my dyno chart with the A/F reading:

 

yeah man 980 is way rich and i bet it was the fuel trims. if you can tune the VE to get them in line your WOT air fuel will get better then you can get the PE tuned right.

 

Great, thanks a ton for the quick replies!

 

i have a problem with my car getting crap mileage and my ltrims are positive, and my o2s read .890 or so at WOT, like was said, i thought its a lean condition, and it overcompensates with fuel. My car is fine also except the 10mpg. new fuel filter, plugs,wires, o2s are like 5k miles old. im lost!

 

After some more diagnostic stuff, my tuning guy narrowed my issue down to the catalytic converter getting too hot too quickly. He turned off whatever feature it is that throws fuel into the motor when the cat gets hot, and everything levelled off right where it should be.

Looks like those new headers/Y I got yesterday came in just in time