I have recently noticed that the coolant in my 1985 190e is almost black. I flushed the system and since then I have noticed the coolant has looked black almost like oil is in the system. Is there a way to test if it is oil and to find out where it is coming from? (From lines for transmission into radiator) There does not seem to be any coolant in oil or transmission fluid. Also, is there a limited slip differential available from another model that will fit my car?

Black waxy/tarry stuff is very likely oil. Especially if it accumulates on the top of the overflow tank. 190e 2.3's notoriously leak between the oil return and water jacket on the last cylinder, in the head gasket. The gasket decomposes and often corrosion is present on the block. If you do the gasket job yourself clean the block with a straight razorblade..then if you are carefull you can fill in any corrosive defect with a little JB weld.

I have a similar issue with the same engine and MBDOC has posted some handy advice,refer to page 2/3 and look at post from hamish.
keep me informed on your progress.
HAMISH

MBDOC
 

hammish, what is MBDOC and where is it???

I assume it is:

http://www.peachparts.com/shopforum/tech-help/8154-oil-water-m102-2-3-motor-89-a.html

search function is great.