Last Sunday when I was up visiting my folks in Birmingham steam started pouring out of the right hand side of the bonnet and the temp warning light came on, then the following sequence of events took place:

• AA man attended, refilled water tank with water/coolant, then noticed the water pouring out of a crack in the inlet manifold/gasket. We managed to limp the car across town to where my dads mechanic lives and left the car with him. (I had to travel home approx 2 hours away to go to work)
• Mechanic fixed the leak, drove the car around several times to check it was all ok, and in checking over the coolant system noticed that the theromstat wasn't working and he couldn't get the fan to kick in, but the car seemed fine.
• My dad drove the car back across town to my mums house for me to pick up (approx 6 miles). After he drove it across town (with no problem) he fitted a new theromstat and left the car for me to come and pick up.
• Picked up the car, had driven less than a mile from home, temp light came on, on opening the bonnet, no leak but you could see air bubbles in the water tank, and when I released the cap water came flooding out pushed by the air.
• Next day I took the car to a local garage, mechanic there spent all day looking at the car but couldn't see anything wrong. No Leaks, thermostat looked fine, everything else was fine, but any time we drove it, after less than 1 mile the temp light would come on and you could hear the air bubbles and water bubbling round the system.

At this point the mechanic and my dad came to the conclusion that the car had definitely been fine before the new thermostat had been fitted, so the mechanic 'broke' the new thermostat (he took something out of the centre of it??) and since then the car has been fine.

I drove the car home yesterday, and to and from work today, no air bubbles, checked the water level after each drive, and no change in water level, and no overheating.

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My current opinion is to leave the thermostat broken and not to get another new one, but I don't know how dangerous this could potentially be??? As far as I know the old one had been broken for a while (I have had the car for just shy of a year) and I have had no ill effects.

Can anyone suggest why a new thermostat would cause the car to overheat so quickly and cause so much air in the system?

Is it possible a brand new thermostat could be faulty, or could there be another part of the system that is broken, and having a working thermostat is causing that part to make these problems??

Thanks