coolant problems - not overheating though

[adrian]

Hi All,

I've just replaced the thermostat and its housing becasue I had a coolant problem that still remains. The car doesn't overheat and the temperature gauge is dead centre. However when I come to a stop the coolant seems to boil in the expansion tank. When cold the expansion tank is practically empty - if I fill it to the 'cool' level it will boil even more and flood out of the overflow. There is no evidence of oil and water mixing and so I'm pretty sure the head gasket is ok. Anyone experienced this problem? I've just drive 100miles with no problems, including sitting on the M4 for 30mins in a queue and throughout the journey no problems... when I pull up on my drive and switch off the expansion tank was overflowing!

Is the radiator in need of replacement?

Thanks in advance for any advice offered.

Regards,

Adrian

[jitsjaf2009]

I would start with draining and flushing the system. Try flushing from every direction to ensure there is no blockage. Refill with 50/50 coolant (I like Prestone G05 if I cannot get the BMW coolant right away...others swear by DexiCool). Take your time filling the system and ensuring you have no air in the system. I "massage" the upper and lower radiator hoses as it warms up to get as much air out of the system as possible. I also start with the reservoir cap off until it is almost operating temp. Go through the "topping off" cycle several times to ensure all air is out of the system. I have found that BMWs and Saabs both are pains to get air out.
When it is overheating and venting, are both upper and lower radiator hoses hot and are both sides of the radiator hot?

[adrian]

Thanks for the advice. The problem turned out to be two separate issues. Firstly the viscous coupling wasn't working properly which led to overheating but not to the extent that the temperature gauge registered increasing temperature. This lead to the sealing cover over the level sensor (my car doesn't have one fitted) cracking and hence under increased temperature so the crack opened up and coolant leaked. The boiling was due to the viscous coupling not being as effective as it should be. Replaced the viscous couple / fan and the radiator and all is now fine. Tip here is even if the viscous fan appears to be working under increased revs is when it really does its job and its then when it might not be as effective as it needs to be. If in doubt and For £50 and 30mins of effort its worth changing....before you too knacker your radiator and then its another £100. Thanks to all that have looked at the post and I hope this helps others in the future!

[jitsjaf2009]

Thanks for sharing the solution. I can't believe I didn't think of the Fan clutch...I think I check it just about everytime I am under the hood. Thanks again...jim