For me this has been Summertime SOP for years and years:

1)Remove thermostat..... 2) Flush the cooling system 2 or 3 times, finally running 95% straight water with just a little coolant to prevent rust....... 3)Install thermostat and add coolant in the Fall.

*footnote: the most effective way to flush cooling system is to remove thermostat first.

Haiges - MB manual is not gospel.... For example, they've got that idiot fan shroud that should be ripped right out of every car - No, it doesnt make the car run cooler for its being there.... yet it discourages ordinary maintenance and repair, creating huge hassle (just in case someone is stupid enough to jam their hands into fan blades)...... Also, just for grins, ask your dealership about putting a trailer hitch on the 4-spd. 240D. They will crap in their pants!! Sorry, but the MB manual is not the gospel.

Quote: "As long as the coolant doesn't boil, an engine won't be damaged buy running hot. Look where the red mark is on the temperature gage. "

This may be OK for you, but not for me.... aint never gonna get even close to overheating my engine - and i think on anyone who DOES overheat an engine as being incompetent for not watching the guage and not taking preventive/maintenance measures first.

Stevo - If your theory is correct, then logically a fully open thermostat also prevents the radiator from getting any water. If anything I'd expect the opposite to be true - whereby closed thermostat cuts off the radiator, channeling water through the engine instead. I dunno much about the radiator bypass system, maybe you can explain it.

Experience teaches me the '79 240D runs cool as cucumber at/near 175* without thermostat after effective coolant system flush. And warmup time is not outrageously lengthened either.