Oil residue in coolant expansion tank – typical sources?

Our 1990 740 turbo, 138,000 miles, has oil in the coolant expansion tank. The volume of oil is small, not a layer of oil on the water, but rather a thick coating of the tank by a very dirty oil. The coolant was flushed and refilled with new coolant in September. The engine oil is clean and free of water.

Are there typical sources for such small amounts of oil?

The only recent problems have been a very rich running engine (poor MPG) and the removal and testing of the temperature sensor for the ECU.

Thanks

Curt

You can test for atf leak into coolant by introducing a flourescent dye into the atf. After a week or so shine a uv penlight or other uv light onto the coolant...if the oil slick glows, bingo.

Check to see the coolant glow before you add the dye just in case your coolant or the atf glows without the tracer. Also, this won't work if you've already put tracer in the engine oil (you may have slight leak with head gasket and we're testing for atf cooler leakage).

One quick check...pull your atf dipstick and make sure you don't have mousse/mayonnaise in the atf. If you have mousse on the dipstick...immediately deadline the car, replace the radiator, and flush the automatic transmission with about 15-20 quarts of petroleum atf (synthetic too expensive to risk it just yet). If the transmission lives another 5000 miles then you should flush with synthetic. I like to drain the atf pan every 20K and top up with new synthetic mainly because I'm too lazy to hook up the flush lines unless I have to!

You can test for atf leak into coolant by introducing a flourescent dye into the atf. After a week or so shine a uv penlight or other uv light onto the coolant...if the oil slick glows, bingo.

Check to see the coolant glow before you add the dye just in case your coolant or the atf glows without the tracer. Also, this won't work if you've already put tracer in the engine oil (you may have slight leak with head gasket and we're testing for atf cooler leakage).

One quick check...pull your atf dipstick and make sure you don't have mousse/mayonnaise in the atf. If you have mousse on the dipstick...immediately deadline the car, replace the radiator, and flush the automatic transmission with about 15-20 quarts of petroleum atf (synthetic too expensive to risk it just yet). If the transmission lives another 5000 miles then you should flush with synthetic. I like to drain the atf pan every 20K and top up with new synthetic mainly because I'm too lazy to hook up the flush lines unless I have to!

Typical sources:
-leaking transmission cooler in the radiator (is this the original radiator? If so, it needs changing anyway)
-leaking headgasket

More likely is number one.