M103 Stumbles off the Line
 

Just lately after the car has been running for 30 minutes or so it stumbles a little starting from a red light. I'm assuming its an ignition problem, and not mounted on the engine since it takes longer for the engine to warm up before it stumbles. It's a 1991 190E with a 2.6 liter M103 with a 5 speed standard transmission, and has a newer (since December) distributor cap.

what could I check and what could it be?

Thanks

Did you also change the plugs and wires when you changed the distributor?

Spark plugs: yes
Wires: no

It seems like all cylinders are missing so I assumed it would be something common, not those.

Check o2 sensor. Then check airflow potentiometer. Check TPS. Check egr system. Check for vac leaks. Check evap system (fuel tank vapor recovery). Check IAC. Then check duty cycle.

The o2 sensor is new (it was smogged in December).

What's a TPS and IAC?

TPS Throttle position sensor. Test at throttle no load and full load. Easy to test. IAC idle air control valve. More "organic" to test.

Here's the W201 FSM link

Untitled Document

Just in case you don't have it.

I think you'll most of what you want in the engine electrical section

http://www.startekinfo.com/StarTek/outside/11832/Resources/201Create/PDF/50008.pdf

Don't forget something as simple as the correct idle speed and new fuel filters...

if it is happening after 30 min it is a hot problem only. I would check the OVP relay it sounds like a bad one they do that when they get hot.

Also test the o2 sensor in series with a multimeter. The o2 sensor doesn't think, it just give you a number. This number tells you what your air/fuel mix is. The computer thinks and reacts based on the o2 sensor value. Easy test to do, yielding very useful info.