New Member and idle only when in drive gear - PeachParts Mercedes-Benz Forum

bobkat · #1 01-03-2005, 01:10 AM

Hello everyone! I just bought a 1988 300E (with 122k miles) two days ago to replace my 1982 300SD (which has accumulated 274k miles).

I am slowly sorting out all the problems with the w124, but i am still having idle issues. On the first day the idle was all over the place, very incosistent. I did new plugs, wires, distributor, and fixed a small vacuum leak and now it idles like a purring cat at about 700 rpm.

The Problem: With my foot on the brake I move from N or P into D or R and the idle drops to 500 rpm and the car becomes rough. If i'm in park or neutral with or without the brake the idle is smooth.

Is it normal for the idle to drop from 700 to 500 (i have never had a car do so), and what do you think my problem is?

I'm new to the w124 world, so i bare with me if this is a simple easy to fix problem, i used search and couldn't really figure anything out.

EDIT: i just found my other post about this problem!! I thought for some reason it hadn't gotten posted so i posted again ... sorry hope this doesn't inconvenience anyone.

olpos · #2 01-03-2005, 01:32 AM

welcome to the gas powered cars. i had the same question on my 88 124. my car has always idled at 500rpm. with a rough idle , worse during the summer. as far as i could find out this is a normal idle on a lot of these cars. what finally fixed the roughness was motor mounts(cap,rotor and wires made a small change) and what a diferance it is. this car runs like a brand new car.
plus the parts were cheap and very easy to put in. did all three in one hour.
best money i have spent on this car
good luck
rich

haasman · #3 01-03-2005, 01:54 AM

Typically these older cars have air leaks and they often come from the tubes connecting to the idle control valve. Remove the air cleaner assembly and start checking the tubes. If they are hard and brittle, replace them. Not very expensive.

Also check the idle control valve while you are there. Often full of crud. Clean it out using something like a carb cleaner until it is free of the built-up mess.

Check the engine mounts also. On a car of this vintage they tend to collapse and transfer even minor idle irregularities into the car. Tip- often only the right side has failed because of exhaust pipe heat. This is good news since it is easy to replace. The left side is quite a bit of work because of the intake manifold above and the proximity of the steering gearbox.

A quick test is to see whether you can get fingers in between the cross member and the rear portion of the oil pan. If it is a tight fit you need to do them.

Keep us posted,

Haasman