Rough Idle ! 2000 Pontiac Sunfire

OK here we go........2000 Pontiac sunfire gt 2.4 litre engine 180 000 Km
rough idle started suddenly one morning......
things I've tried and tested.....
changed spark plugs twice
replaced throttle body gasket
replaced intake manifold gasket
tried new coils
checked engine compression all between 216-222 psi
checked engine vacuum not steady but between 14-15" mercury
added two cans combustion chamber cleaner
no check engine soon light!!!
my GM mechanic had it for three hours could not find anything
added one can injector cleaner

thing i haven't checked
evap
egr
catalytic converter
fuel pressure

engine idles rough only, when it gets above 1500 rpm it disappears but it takes longer than usual to come back to idle speed , all this happens hot or cold.......

Hope you can help or suggest............Rick

Response:

Your problem you say is below 1500 rpm and that the vehicle runs fine above 1500 rpm. I would say you need to replace the IAC valve (idle air control). The IAC only functions at or below 1500 rpm. This is a common issue with GM engines. If you have replaced the intake manifold gasket , this should be fairly simple for you. Also when you remove the old IAC clean any excessive carbon that may be present.

my car will cut out while driving in all gears at all rpms

Question:
I have a 1994 mustang gt 5.0L 302, my car will cut out while driving in all gears at all rpms and when i push the clutch in it will rev itself to over 2k rpms and wont drop till i tap the throttle then when it drops it will only drop to 1500 then will drop to normal when i am stopped idling for 15-20 seconds. Already Tried: I cleaned the idle air control motor

Answer:

This high Idle is a classic description of a vacuum leak. Most common place for the leak is the PCV valve hose located at the back of the engine under the upper intake manifold.

Rough Idle

OK here we go........2000 Pontiac sunfire gt 2.4 litre engine 180 000 Km Rough idle started suddenly one morning......things ive tried and tested.....changed spark plugs twice replace throttle body gasket replaced intake manifold gasket tried new coils checked engine compression all between 216-222 psi checked engine vacuum not steady but between 14-15" mercury added two cans combustion chamber cleaner no check engine soon light!!!my GM mechanic had it for three hours could not find anything added one can injector cleaner thing i havent checked evap egr catalityc converter fuel pressure engine idles rough only, when it gets above 1500 rpm it disappears but it takes longer than usual to come back to idle speed , all this happens hot or cold....... Hope you can help or suggest............Rick

Answer:
Your problem you say is below 1500 rpm and that the vehicle runs fine above 1500 rpm. I would say you need to replace the I A C valve (idle air control). The I A C only functions at or below 1500 rpm. This is a common issue with GM engines. If you have replaced the intake manifold gasket , this should be fairly simple for you. Also when you remove the old I A C clean any excessive carbon that may be present.

Let me know of your progress.