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Tom 10/16/2020

2010 BMW 528i Base 6 Cyl 3.00L


How do I know if the thermostat is bad?

The car will warm up, the cooling fan does not come on, then the overflow expansion tank builds pressure and bursts. I have been through two expansion reservoirs already. I believe the cooling fan does not come on because it's thermostat related. I don't know where to go from here.

1 Answer


Jimm 10/16/2020

There are a couple of possibilities:
- overfilled system(too much fluid in expansion tank; level too high) will not allow room for coolant to expand when heated to operating temperature, thus over pressure
- head gasket leak/combustion gases forced past leaking gasket into cooling system passages in the head = increased pressure/over pressure in cooling system.

It can be confirmed with a simple chemical test for CO2 in the coolant


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