My 92 300E (2.6L) starting running rough the other day, like it is missing. This was not a progressive problem, but started to do this one cold rainy morning a few days ago. The car shakes at idle and light acceleration, and seems to be an engine miss and not just bad motor mounts (although I am not ruling anything out).

I have replaced the plugs, the distributor cap and rotor, and had replaced the wires about a month ago, so they should be fine. Have looked around for any vaccum leaks and have not found any. I also replaced the OVP relay, with no change. I did a compression check and all cylinders are 130-160 psi.

What else? The miss is very consistent, and doesn't go away under any conditions (engine cold or hot). Car fires right up when cold with little hesitation. Could it be bad fuel? Bad injector?

I think this is unrelated because I believe it did this before the problem above, but when it idles, there is a looping as the RPM changes just slightly every 4-5 seconds, like something is cycling on and off. At the same time when this happens, I can hear a click coming from inside the driver's door, like a relay or switch, and it conicides with this looping. Very strange, as I am not aware of any relays in the door that would click intermittently! This may have nothing to do with the engine miss, but when you start really paying attention to details, you notice things like this.

Looking for help...thanks.

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K