BMW: Overheating, needle gauge, clutch fan

Question
93' BMW 316i:
A week ago the car was left idling for about three quarters of an hour, steam started coming from under the bonnet and all the water and coolant from the radiator had all leaked out under the car. I was away from home so i refilled it with water and coolant and left the car switched off for about four hours. Went to drive it home, the needle gauge for the thermostat went right up into the red, i pulled over and stopped the car, could hear water boiling. Left the engine of for a while and drove it to a nearby village (needle of thermostat gauge was in the red for about two minutes and then went down as i drove the car but not much, just clear of the red) and had it towed just incase i was doing damage driving it. The car cut out in the village anyway. Brought it to a mechanic, he took out the thermostat, he said it was gone and was stopping water getting through to the engine so he took it out and told me i could drive the car for small journeys but to keep it topped up with water. He said to get it checked as the head gasket could be gone. Brought it to another mechanic, he's putting in a new thermostat and clutch fan this week to see if that will fix the problem(existing one spins freely when car is switched off). Drove it 12 miles this weekend, kept water topped up, steam started coming from under the bonnet again, i stopped and refilled the radiator, had to fill it five times on the way home, i'd fill it leave the car off for a half hour and then drive it again and it would start again about three minutes into the drive. Needle never went into the red, i parked it up when i got home and looked under the car as the engine was running, it was leaking water big time. Put it in the shed went out this morning, filled it with water turned on the engine and no water is leaking. How do i know if the head gasket is gone and why does it leak water while running and then stop, could it be to do with the heat of the engine. I've priced around and because its a timing chain on the car the prices im being quoted for doing the head gasket and thats assuming the head isnt cracked and doesnt need replacing are seriously high. I need advice, as in what could be wrong and what to do. Sorry bout the length of the message. Thanks in advance.  

Answer
Welcome to BMW cooling system hell...

There are a number of factors that can cause what you described to happen.  IF the head gasket was gone you would have some kind of water in your exhaust... In my opinion you have a leak somwhere in your cooling system. The places I would check is where the coolant levle sensor attaches to the fill tank on the radiator. Another problem spot is the top of the canister itself (below the radiator cap)... A pressure test can be performed to find the exact location of the leak or if you can cant visibly find where the coolant is leaking from.  

It sounds like the bad thermostat disabled the coolant from circulating and thus causing extreme heat and pressure in the system... the weak (plastic) points would go first and with a leak the system will poorly pressurize this causing constant problems.  Also How is your water pump? The factory pumps use plastic impellers it could very easily be that the problem is with your waterpump as well.

Hope that helps.

Manu