Just finished replacing plenum gasket and intake gaskets per Chrysler/TSB/AllData Instructions. 1999 Ram Vam 1500 5.2L Vin Y. Many problems resolved except I still have a high idle after engine is sufficiently warmed up. Idle is around 900-920. It eventually slows to normal (sometimes) when left in Park for a while. I suspect a sensor is telling the Computer to increase the idle but don't know which sensor it might be. IAC seems to be OK. Runs much better now that the plenum is no longer leaking and sucking oil vapor into the intake (duh). Has anyone seen this specific problem before??

 

Assuming you don't have a vacuum leak...a bad coolant temperature sensor will increase your idle. Not the one for the dash gauge, but the one for the ECM. If the ECM thinks the engine is cold, it increases the RPMs to help warm up the engine faster. If the ECM never gets the warmed up signal, it never drops the RPMs down. A bad IAC will usually cause surging at idle, not a high idle.

 

I think I remember mine took a day or two to settle back down after the plenum-gasket replacement.

 

I agree with smartobject. The PCM needs to re-learn the change in the engine. If it keeps this way after a 100~200 miles, then look at the ignition cap-n-rotor. Make sure they are copper/brass and not the aluminum cheap-o type.