Thanks to all of you who replied to my previous posts. The stuff in my coolant overflow tank is black, not red, as I would expect ATF fluid to be. The oil in both the engine and the trans does not appear to be milky or diluted. However this goo is getting into the cooling system, it does not appear to be a two way transfer.

So, if I understand it correctly, this stuff in the overflow tank could be:
1.) Oil, from either a leaking head gasket or an oil cooler (even tho I don't think the oil cooler and the radiator share a common line, right?)?

2.) ATF from a leak in the radiator (although this should be a red colored fluid, right?)

3.) Oil from a "one way" head gasket leak?

So, how do I tell what it is? What tests should be performed? Any advice is much appreciated.

Thanks!

Gavin

1990 745T
1976 2002
2002 330i

Years back I had a similar problem.It turned out that the turbo housing was cracked and when really hot it would allow oil to go into the cooling system.It took me about 6 months to figure it out. In my case once I changed the turbo the problem dissapeared.Good luck.

John has a good point about hose deterioration. The hose goo should be sticky, not oily, perhaps with some particulates. Oil and coolant is usually brownish in color, not black (unless you haven't changed your oil in about five years!) and is quite slippery.

I can't stress how important good cooling system maintenance is: hose, radiator, water valve are all wear items. Replacement no longer than every eight years is very important for engine life.

....."The stuff in my coolant overflow tank is black"....

The black goo is from the inside of the hoses. It is long past time to part with some Euros for some new hoses. You have been too cheap for too long. You were ready to buy a headgasket when all you needed were the new hoses which you have been procrastinating on buying.
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john