These can be a real pain.

Does your car fall into the age range where there are serious wiring harness problems? If so, does it have a new harness? The insulation falls off the wires, causing all sorts of strange behavior.

If this is not the problem, you must chech ALL the vacuum systems (including the climate control) for leaks -- idle control system, brake booster, tranny modulator lines, climate control, EGR if you have one (too new of a car for me to know off my head on that one), etc. Anything larger than a very small leak will cause over-correction from too much unmetered air getting in.

Once you pass the ten or twelve year mark, the rubber parts of the vac system in the engine compartment start to get rock hard and leak, causing trouble.

Also, check the seal on the oil filler cap -- they get hard and leak oil one way and air the other -- new cap is a couple bucks.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!