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Rev happy 300M

My 2000 300M has 135k miles on her. With the a/c on when I come up to a traffic light and stop, it feels like i have to press harder then normal to keep the car stopped at the light. the other night at a light I put the car in nuetrel and the engine rpm went from 700 rpm to 1500 and up to 2000 rpm. There is no check engine light at this time. the car does not to this when the a/c is off. I have not scanned the car yet. any one have any ideas on what is going on???

All cars have a way of boasting the idle rpm just a bit with the AC on. Your car has a problem in that system. Could be a ECM, or a sensor not functioning properly. Since this is not an emission problem it might not set a “code”.

I’m not familiar w/the working of your car, but many cars use a gadget to boost the idle speed under certain conditions – headlights on, AC on, coolant temp low, etc – the gadget is called the IAC (Idle Air Control). It does exactly the same thing as if you pressed your foot on the gas a little. I expect either this IAC of yours isn’t working correctly, or as UncleTurbo says above, the electronic signal from the ECM going to the IAC somehow has stopped working. I’d start with the IAC. It might be the connector has come loose. Or other’s here have said the IAC can get unwanted stuff in it’s inner workings, which causes it to stick. If that’s the case, a good IAC cleaning may be all that is needed.

The only other thing I can think of is an air leak which allows unmetered air into the air flow inlet to the engine. Such as a vacuum leak. Atart with the IAC, and if that isn’t it, look for a vacuum device leaking.

The IAC is used anytime the engine is at idle, whether the AC is on or off. On some cars, the ECM does increase the idle speed a little, but going to 1500-2000 rpm is not normal. There is a code for high idle, but usually it has to detect a high idle for a certain time period, not just a few seconds.

The IAC could be getting stuck when the ECM commands it to open a little, and that would be the first thing I would check. It might have some carbon build up on its valve stem. I would also look into a dirty throttle body that is not allowing the throttle to fully close, but that is less likely to be the source of the problem.

Keith i think you hit the nail on the head. Now i have a place to start looking. Thank You

Give George a little credit too.

If that fails start looking at the vacuum lines, especially close to hot engine components that due to cracking or loose fittings would be good candidates for replacement.

i did clicked on the agree button for George again Thank You all very much