Engine Cooling problem
1985 Other BMW Models 6 cyl Wheel Drive Type unknown Automatic 140 miles
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My 635 csi bmw drives for about 20 minutes, but then over heats and shoots all the coolant out of the reservoir. i just changed the thermostat and the water pump isnt too old. what else could it be?

Try bleeding the cooling system don't work do the tests below.

Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.

Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.

If the block and pressure test passes check the following: Thermostat,Pressure test the Rad. cap,Clogged radiator,Fan clutch,Radiator electrical fan,collaspe hoses,water pump.