Strange Idle problem on 97 E320
 

When the car is warmed up, if I rev the engine in either park or neutral, the rpm goes up and then down and stay around 1500 rmp and will not go down below 1500 rpm. If I shift to drive or reverse, the rpm will be back to normal back down to 700-800 range.

I don't think this is normal. I did just passed the CA smog with no problem recently. Anybody got any ideas?

Is the idle surging at all? Sounds like a MAS on it's way out...

Thanks for reply John.

No, the idle is not surging at all. I did have a check engine light come on a couple of months back. Mechanic read the code and found out one cylinder was misfiring. Did a tune up (replaced the spark plugs and plug ends). Check engine light came back on and then disappeard a couple of days later and the check engine light has not come back for a month and a half now. Don't know if this is related to the high idle in park and neutral.