My car started doing this yesterday. After driving home to pick up some things, I hopped back into my car about 40 min later. I started her up, and while I was in park, I noticed my car running at about 800rpms. Then suddenly it started to rev on it's own, going from 800rpms to about 900, then back down. It did this about three times then stopped. Then happened again a short time afterwards. I drove the car with no problems though. And this morning I didn't see any problems. It was running at 1200rpms.

Is this normal? I've never seen this happen on an auto or manual car before at such low rpms. I'm thinking maybe the spark plug cables are starting to go?

 

Im gonna just guess at this one. So dont take my advice by any means. Just here to help. My guess is the high rev in the morning is due to a cold start. Very common the engine revs a little higher 1200 sounds about right to say the least. Then once it warms up it will slowly go to its normal idle. Which my guess would be 900rpms? The 800 rpm to 900rpm im guessing would probably be your ac compressor clicking on and off? Not sure. It should turn on idle at about 800 rpms for about 15-30 seconds and then eventually the compressor will click off sending your rpms to 900. In other words robbing your engine of horsepower. However to ensure your getting proper spark current, plug wires would be a good start i think your car maybe equipped with coilpacks which run a little bit more than wires. Maybe im wrong there too. Just thought id throw my .02 $ in. Then agian its hard to tell without a car infront of you if you get what im saying. More info would be good though. Is it like a missfire? Does it spit or sputter? shake?

 

Thanks for the input 05cam. I haven't experienced the revving since that day. My car runs fine, no misfire, no sputtering. It was just just revving on it's own. I didn't have the AC on, so not sure if it's the compressor. I do however, have a small leak in my exhaust system. The lead-in pipe to the catalytic has a worn out gasket. Fuel economy has gone down a bit, and a little noisier when I'm driving, but not a problem. Will be taking care of that soon. Could that cause a slight fluctuation in the rpms?