-20% fuel trims on stock engine?!?!?!

2005 GTP comp G

While doing a routine tune up my friends car I decided to scan it. At first things looked normal but then gradually the fuel trims went waaay negative. I also noticed misfires when I stab the throttle and let off quickly while at a dead stop. #3 cylinder misfires the most with # 6 being second (scan shows #4 on start up). I've exhausted myself trying to figure this car out.

At first the car ran like crap because the egr was stuck open. After pulling it off and cleaning it commanded % matched actual % so that's fixed.


Here's a list of things I've done

- replaced plugs/ wires/ air filter/ pcv
- cleaned maf and TB
- disconnected maf to see if trims changed which they didn't
- swapped 2/5 coil with 3/6 (misfires did not follow coil)
- tested fuel pressure (60 psi)
- fuel pressure leak down test did not show significant loss of pressure
- swapped O2 sensors to see if trims changed.......nope
- check for vacuum leaks
- unplugged evap
- reset trims

The only thing I haven't done is plug the wideband in to see what the a/f looks like and swap injectors to see if the misfires follow. Not sure if the misfires and trims are related since there isn't a direct correlation between the negative trims and frequency of misfires. The strange part is, within the first couple minutes of starting the trims look almost fine. IS THIS NORMAL??

what the heck is wrong????

-leaky evap? bad injectors? retarded car?

the scan doesn't show -20% trims but I've seen up to that previously.

Have not looked at log, but have you checked to see if Cat is plugged?

No I haven't, seems to be ok and the logged cat temp is acceptable. Would a clogged cat cause fuel to be pulled?

Well I would assume if you block the flow of air that can exit, you are choking the engine. So it wouldn't need as much fuel.

If you block the intake limiting air flow in, you dont need as much fuel. This seems logical to me that it would work both ways

If it's intermittent it sounds like the Evap is playing it's games.
try unplugging the electric and quick connect (tank side) end on the evap sensor on the engine and see if the settles the LFT's down, Quell the OBD code. then give it a try.

I unplugged the electrical connector but didn't mess with the actual fitting. When/ if I see the car again I will try that.

take a look at this.
http://www.regalgs.org/topic/52807-f..._fromsearch__1

cool, interesting read. However, this car isn't throwing a code. I wonder if this sensor is on the 04+ cars too.