Expert: Josh Fields - 11/16/2005

Question
Hi Josh, I had to change the composite thermostat housing because I had a leak from the bleeder screw area. After changing it out with a factory part I let the car idol until it got up to temp. Their are no leaks, I went around the block and doiing so the temp went way up and the light came on as I got to the garage. My best guess is that I am low on coolant. I am not sure how to use the bleeder to make sure their is no traped air in the system. the coolent bottle is at the full level and I know it lost some during the change and prior with the leak. I did not change the themostat because it never overheated prior to the change. Any help you can give would be great.  

Answer
I'm not sure on the 5-series, but on the late model 3-series with the same engine (I have one) there is a bleed screw beside the coolant fill cap.  Loosen this screw and then poor in the coolant, you will see bubbles rise out of this screw with some coolant.  I am 99.9% sure that you have air trapped in the system, and this will not allow for the coolant to circulate throughout the engine, in turn cooling it.  
 There should be a drain on the bottom of the radiator, and you could drain a gallon from the radiator, in a clean jug, and then loosen this bleed screw while the engine is running, and poor the new coolant in.  Have someone keep a close eye out on the temp gauge, as if it gets to 3/4 to hot, you need to shut the engine off.  
 Make sure you have the heat on inside the car as well, air can get trapped inside the heater core and when you use the heat it will cirulate the bubbles back into the engine.  
 Feel free to ask any more questions!
 Hope this helps,
 Josh