I have a 1989 BMW 525I in Athens Greece that always overheats when I turn on the A/C. I have replaced the water pump,clutch, fan,head,thermostat and all the belts and much more! It appears that when the A/C is turned on, pressure builds up somewhere in the cooling system and the hose to the rear of the engine always bursts. I have vented out all the air more times than I can say here. There isn't a competent mechanic in Athens who can resolve this for me. Is there a blockage somewhere I can't find or maybe a problem in the cooling block behind the heater assembly? Any help would be appreciated. It seems like the water circulating isn't making it through the loop.
Thank You ! Mark

It is possible that you have a large bubble in your coolant system. Check the coolant level.

Is the temperature sensor working fine?

you could have cooked your engine completely, if there's a BMW dealership in athens take it there, , these cars are notorious for air locks, to try bleeding it yourself, jack up the front of the car with the engine cold and off, turn the heater on, and fan on low, open tha bleeder screws, and fill with coolant slowly until bubbles stop coming from the bleeders, leave it for 10 mins, then top up and close the bleeders. refit the cap and check for leaks

Sure I know the answer for this. It's always the radiator, trust me, another good choice is the fan clutch, but rarely the problem.......Bill.