...and if you do a search under his user id for "chip", you get a ton of good reading:

http://www.peachparts.com/shopforum/search.php?searchid=989430

It also appears that were a service bulletin on replacing computers on E420's. Same basic engine, maybe applies to your car:

94 E420 Check Engine Light On

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Depending on the amount of false air being introduced, so would symptoms vary. It is as though you had a carburetor and were adjusting the idle mixture screw. Also, the colder/damper the air, the more oxygen per cubic foot being unaccounted for. It would take less than half an hour to rule 90% of intake related vacume leaks out. An 02 sesnsor does not tune per cylinder rather on overall output therefore you would also likely have the other cylinders running too rich. This would all but disappear after the throttle was opened due to the proportionaly minute amount of false air. On the 129's for instance, a plug reading is almost worthless as they run so lean at idle. I have chased lean stumbles on these cars many times that will exhibit these very symptoms. It is definately worth checking on before anything else. Steve

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64 European Coupe Brilliant Silver with Red interior 4 Speed Sunroof
87 TDT 286k
87 SDL 195k
83 SD 202k
83 SD 201k
83 D 286k
02 R1150RT 32K
00 122HP FLSTF
07 KTM 250SX

"Danger takes many forms"

Sounds like a very likely scenario. I'll check it. Do I need to remove the air cleaner assembly to get to all the surfaces?

I did a bit of reading of the links that were referred to... wow, there are quite a few rough idle problems.

Last night was a chiily night and the problem showed up even worse. I did notice that if I slowed the car down by downshifting, the problem seem to show up when I wasn't at idle. That is, the rpms would oscillate between 500 and 900 when coasting to a stop in 1st gear. When I did finally stop, the engine seems like it's almost ready to stall. If I stop normally, the problem was as previously described. Does this sound like a intake manifold leak or EGR valve problem? (BTW... the problem is the same whether the engine is hot or cold.)

I checked the intake manifold using the suggested technique and found it made no difference. I'd like to check the EGR valve. I do notice it's leaking oil around it. Other than replacing it, how do I check if it's bad?

Check first to see if it holds vacume. Simply apply a vacum pump or you can even get a length of clean hose and suck on it. At idle and with the engine hot you should notice a slight difference in roughness when you apply vacume to the egr. If it does not hold vacume or if there is no difference when you apply vacume, look closer at the egr. They can get carbon caught between the valve and seat. You will only be able to see this via inspection and with the egr removed.

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64 European Coupe Brilliant Silver with Red interior 4 Speed Sunroof
87 TDT 286k
87 SDL 195k
83 SD 202k
83 SD 201k
83 D 286k
02 R1150RT 32K
00 122HP FLSTF
07 KTM 250SX

"Danger takes many forms"

It's 4 months later and I'm still having the problem. What has been checked lately has been for manifold leaks... sprayed around the intake manifold, no difference noted. Checked for vacuum leaks, none found. Tried a new air mass sensor, made no difference. The engine has not EGR valve to check.

I only have a erratic/rough idle that gets worse when it's cool or damp outside. The car has plenty of power otherwise. This problem is costing me a ton of money to try to resolve. The dealer seems to be only able to fix something if it logs a code. I'm now seems down to two things... the coils or bad injectors, as everything else has been either checked or replaced. Any thoughts on which is more likely, given the symptoms? (I'm scheduled to have the injector spray pattern checked this week... should I do it, or save the money?) Thanks!