Having a weird issue that I can't figure out.

2003 Cavalier LS
2.2 L ecotec, Automatic

about 96,000 miles.

I idle at 500 RPM warm - Seems low, but never paid attention to what it idled at before. It is smooth at idle.

However, when comming to a stop, and watching the tach - right before I come to a complete stop and about 650 RPM my dash and my seat vibrate. Its like a rough idle. This is the only time the engine is ever at that speed, but it's very annoying.

Other than that, car is normal. not problems with anything else.

Changed the fuel filter (needed it anyway - thought it was a stretch since it wasn't having a problem at any other speed.)

Took off and cleaned the IAC and butterfly valve - had some good buildup there, didn't make a difference though.

Replaced the EVAP Purge valve - no change.

I'm wondering if I actually am supposed to idle that low, and if so, perhaps a motor mount is just a little worn that it vibrates at that specific engine speed?

I haven't thrown any codes at all - all clean.

Any suggestions?

Thanks!

Forgot to mention,

Transmission fluid and filter also changed. Oil and filter changed.

Does it viberate in park if you hold the rpm at 650rpm, how about in neutral.

Makes a big difference, if it does it only when in motion at 650, versus also shaking a bit in park at 650


- 2004 Cavalier - 124k, owned since new

Yes. It is very difficult to get it to stay a that rpm. But it will vibrate as it goes through that rpm. Does it in park and neutral.

have you replaced the spark plugs in it yet? at that mileage you should be doing em soon... some old plugs could cause a vibration/misfire at certain engine speeds...

I did replace the spark plugs also. (forgot to list that)