I just bought a nice 2001 Park Avenue.  It runs really good except at idle, it doesn't idle smooth and runs at around 800-900 rpm.  I pull the codes and get p0171 - Bank 1 too lean.  The fuel trim is staying around 15-20% and doesn't fluctuate with higher RPMs.  The upstream o2 sensor is reading around .05 and not really fluctuating, so I immediately suspect a vacuum leak.  I check all around the known spots with propane.  Nothing out of the ordinary.  Thinking it might be an internal leak, I shoot the propane into the oil filler neck, and sure enough, the fuel trim goes down to normal and the 02 sensor begins fluctuating normally.


Is this a bad LIM gasket for sure or could it be something else?  Maybe a bad PCV valve?


I've never heard of a 3800 LIM gasket leaking air.


 

pcv  should be  easy to check and is likely oem ... replace it anyways ... I would pull the intake and change the lim gaskets anyways they do have issues at the same time I would go to the help section at the parts store and get the upgraded metal coolant elbows to replace the plastic ones that always fail and break Dorman 47065hp Heater Hose Connector
ETA: every 3800 I ever had to work on leaked coolant at the lim gasket most times on the driver side .. pull it replace them now before you end up doing a head gasket

any updates ?

I'm still leaning towards the lower intake gasket.  I already replaced the PCV and cleaned the MAF.  Fuel PSI seems to be in spec.  I'm going to try cleaning the throttle body before I replace the LIM.