Car: 1995 E320
165K miles

While revving the engine in idle this morning, it became rough just after 4000rpm. From 0 to 4000 rpm, the engine is smooth and imperceptible. But as I ease past 4000, it behaves as if one or more cylinders are not firing in sequence.

Ancillary Observations:

* Check Engine light is on (has been for a long time – 3 years -- with no discernable cause – but I am hoping it is related to this discovery).

* Oil pressure needle, after a fast highway drive, hovers just above 0. Shifting into neutral puts it just above 1.0. (Planning an oil & filter change today).

Any ideas?

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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