I'm troubleshooting a weak idle/stalling in a 98 LS 5 speed, and my datalogger indicates a very high manifold pressure at idle (470-500 mbar). Anybody know what could be causing this? Am I correct in assuming it is a vacuum leak upstream of the IACV?

Edit: I believe the MAP sensor is functioning correctly as it indicates a correct pressure with the engine off.

 

Bump -- anybody know what could cause a very high manifold pressure at idle? I'm not even sure how the car could maintain a low idle with so much air in the manifold.

 

vacuum leak?

 

The valves were too tight. WAAAY too tight. All of them! This is why I do my own work, lol.

 

Yeah I was going to say valves or the chance the cam jumped timing.

 

take it to a better mechanic..

 

It was done by a previous owner.

 

vacuum leak should be pretty obvious. Valve adjustment is another thing and also cam design and cam timing will affect your manifold vacuum.