W201 190E 1.8 Surging idle when hot
 

Hello, I bought a red 190E 1.8 a month ago. I almost immediately had to replace the clutch master cylinder and slave cylinder. Aaand the headgasket was bad, so that was changed amongst the other gaskets on the engine.
Anyways, there's this really annoying problem I have. When the car is above 60c, the idle will often surge so much that the car stalls almost immediately. If the idle is stable for the moment, I can turn the full beams + the extra lights connected and the RPM's will drop and it will start to surge and stall. This does not happen if it's under 60c, but it has happened once just a short while after I've cold started it. It also ONLY happens if the car is at a standstill, will not happen if the car is rolling.
The fuse on the OVP-relay is intact. Anything else I should check?

And there's the problem!!;)

Suppose you expand a bit on the details of the engine: year model, type Mxxx, carbureted or injected, maintenance history, etc.

The OP's car is the same as mine.

M102.910 with the last series KE CIS transistorized ignition

As the head gasket has been off and "other gaskets" have been replaced I'd start by checking for vacuum leaks. Spray something like WD40 over the joints and see if that changes the idle at any place.

You should also check to make sure that all of the vacuum pipes are connected correctly.

Oh yeah I forgot to ask have you got an automatic transmission and / or air conditioning?

It's a 1.8 with KE-Jetronic. M102. 1991. Has about 302000km on it.

Oil, coolant, sparkplug wires and headgasket has recently been replaced. Thought the surging idle was either the sparkplug wires or headgasket (there was oil in the overflow tank). The problem still persist after all of this.

One thing I noticed was that there was no difference on how it ran if I pulled the plug for the IAT sensor. Also tried removing the contact for the ECU and then start the car. The only difference was that the idle RPM's were higher (around 1300) and the surging had stopped.

EDIT: It's a manual w/o AC. The exact same problem was before the headgasket replacement, I doubt there's a vacuum leak caused by that.

The air intake sensor has little effect on mine too. I wouldn't worry about that bit.

Good to hear that the engine runs with out the ECU being plugged in. To my mind that means the fuel side is probably OK. What's happening is that the air idle control valve is not working during this idle phase which means that the high rpms are to be expected.

That you get lower rpms during the warming up phase indicates that the air idle control valve is doing or trying to do its job.

The pipe work that connects the crankcase breathing to the air idle control valve (back of the fuel distributor) and the pipe work that goes into the bottom of the air mixture unit (under the fuel distributor and to the right) needs to be in good condition. Again WD40 might help show some problems in this area.

(Other stuff to consider but I won't over load just yet!)

Thanks for the info. I will check all of this tomorrow, coming back with an answer!

The ECU has a 25-pin connector; perhaps you could narrow your disconnect down a bit.