Hi,

i have just experienced the same problem with my 190e that i purchased a week ago.
When viewing to buy the car i checked the oil filler cap and dip stick but there was no signs of "foam" which made me beleive the engine was fine as it also started and drove excellently.
Unfortunately i didn't check the expansion tank as if i had i would have seen the foamy crap floating on top of the water.
The long and the short of it being that i have had to have a new head gasket fitted at a cost of £500. Gutted.

The moral of the story being "buyer be ware"

good luck and i hope you get it sorted.

Gaz