'91 BMW 520i Overheating

I came back from a 4 month trip, and when I started up the '91 520i, I let it run for a while to recharge the battery. After about 15 minutes, I saw coolant all over the ground. The waterneck from the top of the radiator had broken off and was spewing the coolant out. No damage that I saw (no coolant in oil or vice versa). I hooked up the hose again onto the remaining part of the waterneck of the radiator and refilled it with coolant, but the car is still overheating. I noticed the return hose is very hot, the lower hose is very cold, and I hear boiling from around the back of the engine near what I think is the heater core (just in front of the driver).

I also tried to get the engine to cool off by starting the heater, but it blew cold. Here's my guess...

The thermostat is stuck closed or the heater core is blocked. My guess would be it's the thermostat because of the affect on the entire system. Trouble is, I don't know where the thermostat is. Another option coud be that I have alot of air in the system and just need to bleed it.

Has anyone had this problem? I think the thermostat on a '91 520i is a mechanical one (a round pog with a heat sensitive spring) in line on a waterneck on the engine. Or is it electronic like on many newer vehicles?

Thanks ahead of time...

It sounds like you have air in the system. Fill radiator then loosen the bleed screw over the thermostat. Blow into the fill resevoir (wipe it off first) with your mouth through the fill hole until bubble free fluid comes out. Tighten the screw while you still have air pressure on the radiator. Now loosen the radiator bleed screw. Once again blow into the fill resevoir through the fill hole. Don't blow to fast it will spash in your face. Blow until there are no more air bubbles present. Close the bleed screw while you have air pressure on the system. While you are pressurizing the system you will need to continually add fluid in to the fill resevoir. This is the method I used on my 89' 525 it never over heated again.