I have a 95 325is and the cooling system is driving me crazy. I have read a lot of back posts re: overheating and they have been of some help but I still can't narrow it down. Here's the situation:
Car was kept by my dad for the last three years and driven very little. He had a new radiator put in while I was overseas. Drove it cross-country when I returned to the States this summer with no cooling problems except a lack of heat when I needed it in Breckenridge, CO! Got it home and found that it wanted to overheat in our bumper-to-bumper San Diego traffic. Drained the fluid from the radiator only (couldn't find the engine drain) and proceeded to refill the system, being careful to refill with the heater, get the air out, etc. It eventually took over 3 gallons of coolant mixture (did not use original BMW coolant). Also replaced the coolant level sensor in the expansion tank b/c I'd been looking at the "Coolant Level Low" message for 4 years!
So, things seemed normal for a while, and now at normal driving speeds it runs at normal temperature. However, when traffic slows to a crawl or the engine idles for a while, the temp level starts climbing up, and sometimes hits red. When it does hit approach red, getting off on a side street and ramming air into the front end will bring it back down. It'll run normally in normal traffic after that. Oh by the way, the "Coolant Level Low" message is back with a brand-new sensor in, and the coolant level in the expansion tank is filled to the line!
After reading through this forum, I was suspecting the fan clutch, the water pump, or the thermostat. Both fans run fine at idle. The water pump has never been replaced (117,000 miles), and neither has the thermostat. I was thinking about running up to my neighborhood EZLube for the coolant flush, because they supposedly pressure-test and leak-check the entire system when they do that.
Sorry this was so long, but wanted to get ALL the info out there. I look forward to hearing your answers, and I thank you in advance for your help. Happy motoring,