Quote:
Originally Posted by ENINTY
The water pump is electric and not driven by the engine belt system. The thermostat is separate from the engine block (it's actually mounted to the water pump). Being that the water pump is electrically driven, the failure of it is more related to production anomalities rather than wear. Waterpumps that are belt driven usually fail because the lateral pressure the belt exerts on the pump shaft eventually wear the seals and then bearings. Because the E90 water pump has no belt pressure as mentioned it will not fail for that reason. The only problem with this design is that you can't tell when the pump is starting to fail. Belt driven pumps have a weephole in them which provides indication that the inner shaft seal is starting to fail because it will allow a small amount of coolant to leak out for detection. If the E90 waterpump fails, it will show an engine overheating warning indictor in the gauges.
The thermostat fails open, so it won't overheat the engine if it does fail. Also, the thermostat operation is monitored by the OBDII system, and if not operating properly will trigger the "service engine soon" light.
You should replace either one when they fail. They are not a "preventative maintenance" item.
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Excellent summary. Thank you.