E46Fanatics - View Single Post - overheating and cannot find the cause
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Old 04-23-2012, 03:51 PM   #26
Registered User
Join Date: Oct 2010
Location: Little Rock, AR
Posts: 72
My Ride: 1999 BMW 323i
My problem solved

Legend for newbies;
ET - Expansion tank
WP - Water pump
TH - Thermostat housing

After changing almost everything related to the cooling system, problem still existed. The only two pieces I changed prior to overheating were the TH and WP. I did not want to blame the recently replaced thermostat and I kept changing parts, what a mistake !

However after replacing the already replaced thermostat, I bleed the system according to the cold&hot instructions once again. That's when I realized how the thermostat works. (More experienced mechanics can correct me if I am wrong) The hot coolant is pushed into the radiator from the ET-radiator connection close to ET cap, that's why radiator gets hot close to the ET connection at the beginning, and the rest of it stays cold till rest of the hot coolant fills in. After the radiator is totally filled by hot coolant, (that's when the lower hose gets hot) the temp gauge on the lower hose reads the coolant temperature and the thermostat opens. If you have a faulty thermostat, the WP keeps pushing the coolant but fluid never circulates inside the engine, only between the ET and the hoses. Excessive pressure builds up in the cooling system (not in the engine) and EP explodes (what happened to me).

A useful tip: some owners (i.e. Paraklas) used high-temp silicone which I think is really useful however you don't need to use it all over the thermostat housing area. If you just put a thin layer of high-temp silicone all around the TH gasket, it will stop the leaks. Do not over tighten the screws to stop the leak. Not only it won't do it but also you may snap your screws. When I replaced the TH second time I realized I over-tighten the screws which got stretched and thinner but did not stop the leak. Use silicone, do not over-tighten, be careful!!!

Cold bleeding works if you properly drain the system from the drain hole under the engine. It's close to the engine bracket on the passenger side.

Last edited by Xeus; 04-23-2012 at 03:57 PM.
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