My 300sel is overheating. The resevoir empties but there is no fluid anywhere. Is it a thermostat that has gone bad? And where is the thermostat?

Headgasket is blown...if it was not replaced on that engine before,your antifreeze is in the oil,pull the dipstick and see the sludge,and,dont drive it anymore,otherwise,you will have to pay to junk it.

There is also the remote chance its leaking into the transmission (at least I think I've read about it), but most likely its head gasket time. Find an independent shop with a good track record to do it cheap. Last one I did set me back $1200.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

Yep, replace the head gasket. Expensive at a shop, not expensive if you do it yourself.

Also, depending on the miles on the motor, it might be time for new valves/valve springs/valve guides/valve stem seals....etc...head rebuild.

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