I've been suffering with this problem for months now. Finally the check engine light came on which hopefully will shed some light on the problem.

Initial problem - no CEL:

- Engine usually runs smooth at cold idle. But not for long - maybe a minute or so.
- When at idle engine will alternate between running smooth and then stumbling and slowing down. Seems like the ECU is trying to keep it from stalling. Whole car will tremble.
- Lower RPM has limited power/torque. Once you get to 3000 RPM or so it pulls much better.
+ I replaced all plugs (not coils) and problem seemed better for a few days. My wife was driving it so I'm not totally sure.

Then recently:

- At highway cruise engine will surge rhythmically. Not always, and not strongly. More of a gentle rocking motion.

And most recently:

- Check engine light came on. I got 5 (EGR) and 8 (coolant temp sensor open circuit). I cleared both codes.

What I have done:

+ I tried disconnectig the EGR vacuum line altogether (and didn't cap it if that matters). Didn't seem to make a difference one way or the other.

>>>> While inspecting the vacuum lines I broke the plastic one in front of the engine that runs to the switch that controls the air pump and EGR - it was very brittle but I don't think it was already broken. I capped it. Now the engine always runs rough at idle but at a consistent speed and I can now also hear it in the exhaust note . Power seems to be back above 2000. Lots of it and makes me remember why I used to like this car.

+ EGR has a dot on top of it so probably it was replaced with the new type at some point.

+ Wiring harness has been replaced before I got car and it looks in excellent shape.

+ I cleaned the MAF and another sensor farther down the manifold with electrical contact cleaner. No difference.

I will be repairing the vacuum line ASAP.

I am STUMPED.

Thanks


E320 1995 176000 miles. One of the later in the year ones.

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1995 E320 with 175000 miles and completely failing original Mercedes paint.