I recently had the hose from the head to the heater split on my 91 300E. In looking at the inside of my coolant expansion tank, I am finding gunky deposits that are light brown / tan to white in color, and thick like grease, not oil such as you'd find in dirty dishwater. I was thinking that I didn't rinse all of the flushing agent out last time, which was pretty foamy. It feels kind of like soapy scum, but that could be wishful thinking on my part, slick is slick.

I'm not finding anything wrong with the motor oil, and I can't see where the level has dropped since I changed it last.

Please?

Thanks,

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On the plains of hesitation bleach the bones of countless millions who at the dawn of victory, sat down to wait, and waiting -- died

I don't want to be the bearer of bad news, but oil in the coolant looks just like what you have described. This is a typical sign of head gasket failure on the M103 engine. Coolant generally does not find its way into the oil on these and the engine will otherwise seem fine with no problems in the way it runs. The head gasket will simply have failed between the oil and coolant passage at the rear.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
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I'll have to support Greg in his "bearer-of-bad-news" job. What you describe is the typical symptom of a failed head gasket which is not an uncommon occurence on a M103 engine.
The engine head has to come off and it would be appropriate to have a look at the valve guides at the same time. At least, replace the valve seals while the head's off.
Good luck

Thanks a lot guys. Don't worry about 'being the bearer of bad news', it was much appreciated. So now I need a recovery plan....

__________________
On the plains of hesitation bleach the bones of countless millions who at the dawn of victory, sat down to wait, and waiting -- died