I'm not sure if I'm the Don you're looking for (screen name is ddrake3) but I think I can solve your problem, as it has happened to me. There are a couple of things to check... #1) Check your radiator cap to make sure the seal isn't rotted... if it is, the water in your coolant can evaporate, leading to a low coolant condition. 2) once you know the cap is good, top off the coolant WITH THE BLEED SCREW OPEN until coolant just begins to come out of the bleed hole, to make sure there's no air in the system. Then let the car warm up with the screw open and the heater on max. You'll notice from time to time that air bubbles will come out of the bleed hole. Yes, you can leave the bleed screw out while the engine is running. Once hot air comes out of your heater, carefullly recheck your coolant level (don't forget to open the cap slowly, allowing pressure to escape before taking it off.) I use a heavy towel to make sure I don't get burned by coolant. If hot air doesn't come out of the heater but the engine is warm, rev the engine a couple of times-- that should solve it. Once you've done all this, your problem should be solved. I have a number of posts on the topic on this site. Post again if you need more info. This post can only be 1000 characters long...
1990 300E 125k Miles
1994 S420 100k Miles