Greetings fellow Bimmerphiles, I tried posting this in another forum but got no response so I figured I'd try here. I have a '97 E36 M3 that I acquired almost two years ago. It had already been somewhat modified when I got it. Unfotunately, most of those mods were ill conceived. I've done some work/had some work done on it in the mean time, mostly correcting mistakes of the previous owner. The Air Force trained me to fix F-16s but I'm not too knowledgeable about cars and half the point of buying the M3 was to work on it and learn along the way. Learning about the ins and outs of BMWs is both facinating and mind boggling. Anyway here's the problem. My heat only works sometimes. It has an automatic system and when it isn't working the fan stays at the lowest setting even after the engine has warmed up. If the fan is turned up manually the air flows as strongly as it should and through the correct vents but the air is cold. When it is working the air is gets good and warm and the fan adjusts automatically as it should in auto mode. Here's the weird part. When the heat is working the temp gauge tends to climb to the red. I was thinking it might be the heater control valve but would a bad heater control valve cause both the lack of heat and over heating? I've checked the coolant level in the morning before starting it for the first time and it's at the "full cold" line. I do not hear the tell tale gurgling/bubbling sounds I've read others write about when their heat doesn't work. I have the Bentley manual for the car but the illustrations do not clearly point out the heater control valve. Also, the Bentley manual is fine for telling me how to change parts out but doesn't tell me how to figure out what's wrong with it in the first place. Can anyone recommend a good troubleshooting book and/or a book that describes how the various systems work?