1989 260E
189,000 miles

For some time, I have been plagued by rough idle problems. Electricals (distributor, rotor, plugs, plug wire, ignition coil) are all new or near new. Fuel/Air mixture is correct. Fuel delivery – pumps, accumulator, lines, filters, distributor, injectors, injector seals) are all OK. Yet the problem persists until, hopefully, now.

There has been one common denominator in this mystery: the oil pump and its connected parts.

When the engine stumbles at idle or in any other low RPM state, it is visually indicated by the bouncing oil pressure needle. I have always assumed it was simply a visual symptom of a fuel or air starved engine.

Now, however, I am beginning to suspect that the oil pump or a component of the oil transport system (I believe there is also something called a “Sender Unit”) is actually causing the rough idle.

This morning after the first start, the realization began to hit me as I thought, “I am idling at about 550 – 600 RPM, yet the engine is smooth. Why? Normally it should be stumbling. What is different?” The only thing different was the high oil pressure indication.

We all know that applying a direct or indirect mechanical load to the engine, via the transmission, PS pump (on lock to lock turns) A/C compressor, or alternator measurably impacts engine RPM. It is therefore reasonable to assume that a faulty component of the oil pump/transport system -- or the pump itself -- which is intermittently performing normally then not-normally, is perhaps applying back-pressure to the oil pump, causing an intermittent engine load, and thus the erratic idle.

Because the problem only occurs at low RPM And lower oil pressure, and does not occur at high RPM/High pressure nor at low RPM, high oil pressure, I am tending to believe this is the root cause. (At high rpm, any back-pressure caused by a dirty or faulty component is overcome by the pressure out of the oil pump. At low RPM – assuming warm.thin oil, that is not apparently the case. But low RPM with cold/thick oil masks the problem.)

What do you all think? I don’t know enough about oil systems, but this seems to make sense to me. I am due to get an oil change and new filter in a few days as part of normal maintenance, what else can I check? And what is this oil sender unit? Am I on the right track??

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black