I have a 00 3.4 Monte and had a bad vacuum leak because the elbow for my PCV valve was cracked pretty good. My Long Term Fuel Trims were sitting around -12% to -15% on idle, and short terms were decent around 0% to plus or minus 3%. As far as I can tell, that's running rich, but with a vacuum leak it would be running lean, right?

I just replaced the cracked elbow and I'm letting the ECM clear to see if those numbers stay somewhere around normal in the next few days, but I'd like to hear some opinions on this...
Also... O2 sensor is relatively new so I don't see that as being the issue.

 

negative trims do mean you are running rich and are not consistent with an active vac leak. it might be possible your MAF is dirty and not reading correctly. some suggest using the MAF spray to clean it. what i do is not suggested and actually most say not to do it, but it has worked for me... I use my finger and lightly wipe any dirt or oil off the sensor. this is easy as i do not have the screen in my maf. you have to be extremely careful not to break the sensor elements off as they are very lightly welded/ solder on


another possibly is that the vac line to the fuel pressure regulator is leaking... this would cause more fuel to be sprayed and cause the rich condition.

 

thanks for the info, I'll experiment with that soon and see if it makes a difference.

Aaron

 

The first thing to check would be all your vacuum lines... And make sure you used a gasket/RTV when reinstalling the PCV