Problems with the radiator, water pump, radiator cap, water / coolant flange and batter drainage?

Hi there.

I wanted to enquire regarding some work that my mechanic carried out recently. All issues appear to be resolved now but I need to assess what happened at each stage.

The problem originated whilst I was stationary with the engine running. There was a loud bang from under the bonnet, steam came out of the bonnet, and water leaked out onto the ground.

I had the car towed to my mechanic who, after some time, replaced the radiator and the water pump. (If I recall correctly, he had to return the first radiator he ordered as it developed a crack in it (or so he claimed)). Within a few minutes of picking the car up from him, I was driving along and heard a grating/grinding noise coming from under the bonnet again. Once again smoke/steam started coming out from under the bonnet. When I stopped the car, there was anti-freeze leaking out onto the ground.

I telephoned my mechanic who seemed to think the smoke was just from burning off excess anti-freeze that he had just added. He suggested that the problem should resolve itself in about 10 mins and I could continue to drive it.

The problem only got worse as the engine overheated with the temperature gauge up to the max. I managed to drive it home which was about 5 mins away, stopping and starting along the way. After some delay, my mechanic came to collect the car, applying a temporary fix to the flange in order to drive it back to his garage. He replaced the water (coolant?) Flange and the radiator cap, both of which had broken.

On collecting the car from him, a new problem had emerged which he told me about just before releasing the car to me, which was battery drainage. He seemed to think this was due to the car stereo seemingly not turning off. I took the car to another mechanic to sort this issue out whose diagnosis resulted in disconnecting the car stereo; he considered there was an internal fault with the car stereo. I have replaced the car stereo with a new one and there is no further battery drainage.

I would like to know two things:

(i) Whether the failure of the flange and radiator cap has anything to do with the work of the mechanic who replaced the radiator and water pump - i.E. Whether it could be due to him not testing the car properly before releasing it to me, or a failure to bleed the system after he had replaced the radiator and water pump, or something else he may have done. The car admittedly is about 25 years old (BMW Z3 1998) but I can't help but think that it is too coincidental that the flange and radiator cap should break within minutes of picking up the car from him, particularly if he had checked and tested the system properly, and if the replacement radiator had been supplied with a new radiator cap.

(ii) Could my mechanic have done anything whilst carrying out the replacement of the flange and radiator cap, to cause the problem with the car stereo that then caused the battery to drain. I appreciate that replacing these items may be unrelated to the car electrics. If so, can you offer any thoughts as to why the car stereo could have malfunctioned in this way. The stereo is not new, but it is also not the original stereo that came with the car. It is probably about 10 years old.

Any thoughts / advice you can give would be much appreciated.

Many thanks in advance,
Christian