My 2005 R53 recently has had a coolant issue. This week I was driving on the highway during peak rush hour, so going slower on the freeway then normal, and my coolant temp gauge was reading its max and the coolant temp warning light came on. I pulled over and stopped to check it out. The high speed radiator fan was on and the coolant in the reservoir tank was up to the cap, but I didn't see any spilling out. The service engine light came on and it was a P0118 code (online says most likely a bad coolant temp sensor). The engine temp has never gone above the mid point on the engine temp. gauge.

I re-test drove it today and it drives fine, the coolant temp. stays where it should, but once I park it I notice the high speed fan kicks on and the coolant fluid in the reservoir is completely full again. Could a bad coolant temp sensor cause this? Could it be the thermostat? Water Pump?

I very recently put a new coolant temp. sensor in (1-2 months ago) but maybe it was a cheapo one, water pump is newer, radiator is brand new, coolant levels are as they should be when it cools off (quite certain I don't have a coolant leak). Any thoughts would be greatly appreciated thanks!

 

Just to eliminate the possibility of a head gasket leak, have your local auto parts store check for exhaust in the coolant or purchase a kit to test for it yourself.