Makahveli

I'm still new to the mercedes and this is the oldest car I've owned so far so I don't know whats the problem. Within the last 2 weeks when starting my car and at a dead stop it idles rough where I hear and feel it. But What I don't get is that the car still accelerates fine and the rough idle sometimes goes away when shifting from park to drive, and still comes back and dead stops. Any ideas?

bamberger_1

Rough idel has many many reasons. But there are some checks you can make before seing your buddy at the MB-shop
1) Check the spark plugs whether or not they are clean and dry
2) Check the ignition cable whether you can find some damages
3) have the engine running in P and pull one by one of the spark plugs (wear a thick glove to prevent e-shocks and/or burns from hot engine. If one is pulled and no change in idle is noticed check the plug, the cable and the distributor connection
4) check all the vacuum hoses if they are ok, not briddled or broken...
5) Spray brake cleaner or cleasning alcohol in the area of the injection valves to see if they are still sealed. If not they alcohol or brake cleaner would be drawn into the cylinder and first reduce then rise the idle speed. The same test can be done for any air inlet components to check for cracks drawing extra air.
6) check the cold start injector, if it leaks fuel into the inlet manifold not only during a few seconds when the engine starts. (see the post from Ron in SC)
7) check the temperature sensors for short circuits or broken wires, or wrong readings
just 7 out of xx things, but these are the simple things to be done first.

NorCal124

Have you changed your spark plugs recently? If so did you use platinum? If you did take them out and just use standard Bosch super plugs. When I first bought my car it had a rough idle but when it came time to tune it up, that all went away w/ the changing of proper spark plugs. It turned out the previous owner put Beru 4 prong platinum plugs (similar to Bosch Platinum +4), which are a big no no for any W124.

jrmd01

It might be a long shot....but inspect the motor mounts.

Every year I go through some type of idling problem costing me from hundreds (if I'm lucky) to thousands (don't ask) of dollars to fix.

Here are a brief breakdown of some of the SOLUTIONS to my idling problems...
-OVP
-coil
-distributor, rotor, spark plugs
-alternator
-valve seals
-pulley

bamberger_1

motor mount show when they are shot, by loosing a yellowish fluid and you can find traces along the body of the mount. Rough idle is rough idle no matter what the motor mounts are like.

azednalcm

Just like JR, my guess is the motor mounts.

1991-300E

i suggest you check your oxygen sensor!!!