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.