86' 300E with a weird oscillating idle while in drive and reverse
 

I did a search but did not come up with an answer because my problems are similar but not exactly like the others.

I need help. My 86' 300E is idling wierd "bouncing" between 500-600rpm when in gear (while stopped). The ingition system seems O.K. (it's not misfiring). The car will continue to run and has NOT stalled on me.

When I'm in drive, reverse, L2, or L1, the idle is rough and oscillates between 500-600rpm. When I put the car in park or neutral the idle is fine. The car also runs fine when I'm pressing on the gas pedal. This happens whether the car is cold or warm...there is no preference to the running temperature. If I'm on the freeway and take my foot off the gas pedal the car will run fine. It only occurs when I'm stopped.

Anyone got any ideas what's wrong with my car?

Thanks in advance for any advice/opinions.

Sincerely,

jrmd01@yahoo.com

Try shaking your engine. You could have bad engine mounts.

Look under the car, look and see what the space is between your cross member and the engine above. Generally speaking it should be be least an inch of clearance or more.

Haasman

Dear Haasman,

I don't think it's the engine mounts. The car idles fine in park and neutral. Just when the engine is engaged in gear (D,R,L1,L2) the idle will decrease and oscillate between 500-600.

So far I've changed the following:
OVP
Spark plug wires
Plugs
Distrubitor cap
Distrubitor rotor

Reading the archive post, I was begining to believe that the problem might be the throttle actuator. But someone posted a fact that my engine does not have a throttle actuator, but instead a idle control valve. Other possible source could be the MAS, wiring harness, and vacuum leak. I'd hate to replace these things if they are not needed. Can anyone guide me in the right direction?

Common Symptom
 

Unfortunately those symptoms are so common, it is hard to point out what the problem is, mine turned out to be an air leak under the intake manifold, the gaskets were all brittle so I replaced them, and it was fine. Regular suspects are :
Vacuum Leaks, injectors etc. Contrary to what many mechanics may say I have learned from about 3 merc techies that ignition wires are not really a replacement item, if anything it is only the ends that are replaced if bad. After trying to figure it out myself I decided to just take my car to a good Mercedes techie, and he finally diagnosed the air leak. key word is a good merc techie, I had taken the car to 2 guys who were "merc techies" but they couldn't do anything, all they said was the regular stuff, this part, that part, that part etc etc etc, I was smart enough to know though. Hopefully there is a good one in your area. I don't know about dealers either.

jrmd01,

I also have a 1986 300E and have the exact same problems you mention above except for sometimes my rpms' are showing a little higher and more so when cold. But just like you say I have found my problem to be just as intermitent.

Did you find your solution yet?

Also, what are everyone's thoughts as to the oxygen sensor or microswitch in relation to this problem?

Thanks

Dear JFAZ,

I have not been able to solve my idling problem. Actually, after reading my orginal post, I exaggerated on the RPM. Actually when in park and neutral the RPM is around 700-800, then in Drive,L1,L2 and Reverse the RPM oscillates between 500-800 the car runs rough but won't stall. The problem is intermittent, so I've learn to live with it. I've read on this forum that the problem could be the
1. Mass Air Flow Sensor
2. Idle control valve
3. Actuator (I don't think our M103 motor has this - anyone please confirm)
4. Vaccum leak

The only thing I've tried to do is to spray carborator cleaner - it helped out somewhat, but the problem is still there. I think I have to remove the whole air filter assembly and clean out the idle control valve.

The term 'throttle actuator' usually refers to the cruise control link to the throttle, so you should have one. Very unlikely to cause your problem, though.

You can test the airflow sensor pot by unplugging it. If your idle is then smooth, it is likely the problem. With it unplugged though, throttle response will be sluggish, especially off idle.

Steve

This scenario is known as "surging" (which, if you use this word on a search, you will find threads on this).

It is most prevalent on the W124-series cars, but other models have experienced the same.

There have been numerous solutions to the problem, but none absolute.

The most talked about is a vacuum leak. One of your hoses is split or has separated from a connection causing the surge to occur. This is the simplest (and cheapest) fix.

Another member has mentioned an intake manifold leak that was fixed stopped his surging problem.

Others mentioned the AFM or air flow meter needs replacement. I tried to fix mine this way, but it made no difference.

Still another member had the reverse gear sensor switch for the transmission replaced, and it solved his problem.

When I joined this forum three years ago, my first thread was about the surging problem...and I STILL haven't solved it. Hope you have better luck! ;)