irratic idle 98,3.1

1998, 3.1 litre. 85,000 miles. several weeks ago had intake manifold gasket replaced with fel-pro.(you don,t want to hear about that , for sure ) . also had alternator replaced and ignition wires and plugs. problem = in park without a/c the idle quickly goes up and down ,over and over. it seems when i turn on the a/c it settles down. also noticed car is only getting 15 mpg city with a/c on now . any idea,s on this?

Vacuum leak somewhere, it sounds like. Pcv not seated, possible issue with the intake job. Make sure the ducting to the tb has no leaks or cracks. also.

Vacuum leak somewhere, it sounds like. Pcv not seated, possible issue with the intake job. Make sure the ducting to the tb has no leaks or cracks. also.would a hand scanner identify the problem ?

Only by showing fuel trims going rich as a result of a vacuum leak.

Only by showing fuel trims going rich as a result of a vacuum leak. thanks for the info.

Like Max wrote, scan it. Have the MAF g/S and injector mS recorded at a given RPM, then compare between banks, cylinders, and the ratio of air to fuel. Make sure the engine is at full temperature and in closed loop.

You'll have to factor the results if you are using an ethanol blended fuel.