I've had a rough idle problem for months and I'm at a loss as to what do about it. Here's the specifics...

- It idles rough only when less than 500 RPMs. It gets rougher when it's cold and/or damp outside. Everything else is great!
- I have had the following replaced in the last few months: Engine wiring harness, throttle body, distributor caps and rotors, spark plugs, spark plug wires, engine mounts. None of these had an impact.
- An engine compression check shows compression as good as new. There are no codes being stored.
- The car only has 67K miles on it.

The problem seemed to start after having my enginer steam cleaned, but I also had the engine mounts replaced right after, so it's possible it started before.

Any ideas?

You've replaced everything I was going to suggest.

I'm kind of stabbing in the dark, but did you replace the spark plug resistor boots?

Are you sure they are fully pressed on tightly?

What spark plugs are you using? You should be using plain old copper-core non-resistor Bosch or Beru spark plugs.

The resistance in these cars is in the spark plug end (or boot), so if you have installed resistor plugs, even platinum plugs, you have doubled the resistance.

Also, what about the mass air meters? Your car has two of them, and they can cause strange things to happen when one (or both) are going bad.

__________________
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".

The spark plug wire set comes with the resister boot, so they were replaced. They all seem to be seated properly with the "snap" when installing them. The spark plugs are indeed the plain old non-resistive plugs. An additional comment... when the enginer first goes into idle, it seems fine. It's only after it's been idling for a longer period of time when it feels like it's missing slightly. (So slightly, that you have lay your hands on the air clean when the hood is up to feel it. Otherwise, it seems like it just purring.)

I thought about the air mass meters, but wouldn't that also impact non-idle speeds?

My neighbor across the street from me is a technician at Penske Mercedes Benz in West Covina, CA.

I just have had several conversations with him about mass air meters causing strange problems.

If it were my car, I wouldn't spend the money to replace them as it's too much of a gamble. However, a shop would have known good mass air meters on hand, and could swap them out too see if there's any change.

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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".

What a coincidence... I bought my car from Penske motors. Where are the mass meters located?

They are in the plastic pipes leading to the airfilter housing, or possible mounted in the air filter box itself.

Where do you live? I'm out in Phillips Ranch near Diamond Bar.

PM me if you'd like me to ask my neighbor if he can help you out.

He has worked on my car at a discounted rate in his garage.

__________________
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".