I have a 318ti that blows only cold air when the heater is on (idle or high speed). This problem is compounded by the fact that the car "seems" to heat up too quickly. I say "seems" because, while the water temp needle is in the red (even for a long period), there is never a dashboard warning light.

I have noticed that the fan in front of the engine does not run at idle (cold or hot - I don't know if it runs while driving). I have also noticed that the coolant seems to be going away pretty quick. It was down to the bottom of the reservoir today, so I filled it to the COLD line, drove about 1 mile and back, and it was about an inch from the bottom of the reservoir again.

There was something underneath the steering wheel [down near the hood release] (a fuse? didn't look like any fuse I have ever seen - says "AMP" on it, though) hanging down by a black plastic thread. I pushed that back up into the dash (is this the aux fan fuse?). This didn't seem to solve any problems, though.

I don't know much about this stuff (I am a minister), but I would like to fiddle a little before shelling out the big bucks to the stealership. Any help would be greatly appreciated.

My cars doing the exact same thing. I can't tell where the coolant is going. I do however burp my system everytime I add coolant. I recommend doing this as air will build up in your system and eventually cause an air lock once your radiator fills with air, allowing no coolant to circulate. To do this, remove the small cap next to the coolant reservoir. Remove the reservoir cap. Turn the heater on full blast and allow engine to warm up. While doing this, continue to add coolant until you start to see coolant coming from the vent next to the radiator cap. Watch for all of the air bubbles to escape from the system and continue to add coolant as neccasary to prevent overheating. Once you see a solid stream of fluid venting from the radiator vent, cap it off and fill the reservoir. Your heater should start to work now. It also helps if you park the vehicle on a hill so the front is raised to help the air find it's way out and also massage the radiator hoses to push the air out. Besides that, My fan doesn't come on either. I found that it does come on if I run the A/C. I've checked all of the fuses including the big orange 40 amp one under the dash. I even switched the A/C relay with the fan relay since they were the same part in the fuse box. No luck. I've inspected the coolant for oil and the oil for coolant to see if it could be a head gasket. Thankfully found nothing. Besides that the car runs great. I add about a gallon of coolant/water every week. I cant seem to find any signs of a leak or residue anywhere. I'm sure you have air lock which is why your car is overheating. Try what I said above and that should help, but will not fix the problem. If you find anything out let me know as I'm totally stumped and also can't figure out whats wrong with my car. I'll do the same if I find anything out.

I had the same problem. The Switch for the fan is on the passengerside of the radiator on the side near the top. On my car somebody had removed the switch and plugged it with a bolt, leaving the wires to hang. Check the wires

bumpidybumpbacktodatop

Common problems are air in the system.
A loose cable to the temp control.
Bad bypass valve.

Happened to my ti as well. Ended up being a loose wire.

if you are blowing cold air.. you are low on coolant.. close to a gallon I would say. Time to check for leaks

Just let it sit in the driveway and idle till it starts pouring. Let it get warm so the water pump kicks in and starts throwing some fluid around. You'll be able to tell where the leak is if you're loosing that much. Never let it run in the red EVER no matter how inacurate the stock temp gauge is.

check all of your coolant hoses. like said before you are very low on coolant since the heater is not heating. this happened to me before. i noticed that the coolant reservoir never seemed topped off. i just 'bled' the system while refilling it with coolant and since then all is well. coolant leaks suck because they tend to evaporate fast (if it is a small, slow leak).