Reposting with corrected thread title:
After fixing a surging idle problem by replacing MAF, the car now idles evenly at 550 RPM in gear and 700 in neutral.
The car idles evenly but not smoothly, meaning it maintains a steady 550 but there's unpleasant engine vibration while in drive at stand still. Up until a few months back, the car idled at red lights so smooth you could hardly tell the engine was even on. In Nuetral the idle is smooth. In reverse at stand still the idle is smooth. Its only in drive that there is medium to heavy vibration.
At first I thought maybe there is still a vacuum leak somewhere. But now I am beginning to think that months of driving with a bad MAF may have deteriorated the engine mounts or perhaps the caked up the oxyen sensor with carbon.
I jut cleared the error codes when replacing the MAF and since have driven about 30 miles in 5 trips. The CE light has not come back on.
I would really like to get back to that smooth as glass idle.
One more possibly related symptom - the air pump stays on nearly 5 minutes after start up and sometimes comes on for 30 seconds even after driving 10 miles. I've had CE ligth come on in the past for code 26 (faulty shift delay) My Indie advised to ignore it stating it could be a $300 repair for a problem that was not doing any harm.
1995 E320 with 217K miles

Ferdman replied with how to on mount replacement, but after thinking about it, if mounts were the problem then vibration would be there in reverse also. Looking for additional opinions.

Most likely the motor mounts. As pointed out earlier.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator