lvduke

My wife's '89 300 SE has a rough idle that our MBZ mechanics cannot identify or fix. At idle it regularly seems to skip a beat - i.e., the car shakes mildly at a frequent interval - skipping most often and infrequently stalling - but otherwise running fine. The mechanics say it's OK. I'd like to fix it so that she doesn't stall out alone somewhere. I'm not at all familiar with Mercedes engines. On my autos, I would check the spark plugs, plug wires, distributor, etc. What else do you check for on this over-engineered vehicle? If the mechanics can't find it, what might they be overlooking? I would appreciate any useful suggestions. Thanks!

w123_w126

Sounds like a bad fuel pump relay (I believe 89 was the last year for the relay before it was incorporated into the MAS control unit). Your mechanic should know the test for that. You are also right about the cap and rotor. The 103 engines need good ignition components to run correctly. Often the cap can crack or the oil seal leak fouling the distributor components. Less likely but possible is a failing over voltage protection (OVP) relay. If faulty it can cause the engine management system to not work correctly.

If you think a 300 SE is over-engineered try dealing with an early 90s M104 with a bad wiring harness.

lvduke

Thanks W123 W126! I appreciate your input.
Just had the fan bracket bearings (?) go and now they say the water pump needs replacing as well. Man, this thing is becoming a money pit!

BenzForeman510

Usually its the valve stem seals,they are crap. R&R the plugs make sure there not oil fouled..also there is a mod you can do to raise the idle up to fix some rough idle.you have to locate a purple wire under the fuse box and ground it...also check the aux connections at the battery,make sure they are tight
A decent MB tech should know that if they been around