Mechanic says it’s not the head gasket.

[igosaur]

I had the same problem as you for months and yes, it was driving me nuts as well.

Everything suggested that it wasn't the head gasket, exhaust looked fine, no water in the oil and vice verse.

The first thing I did was change the thermostat and this seemed to work for a while but sure enough, not long after my problems were back. Each time it overheated it would dump loads of coolant in the expansion tank and sure enough, the next morning it was back in the radiator.

Eventually I have come to the conclusion that it was a succession of dodgy hoses. Each time I had an overheating problem I would listen carefully under the bonnet, with the engine off of course, and more often than not I could hear a hissing sound.

I have now repaired or replaced at least 4 hoses and changed the clips on countless others and, touch wood, no more problems.

It just takes a small split in a hose or a badly fitted hose to stop the whole system becoming pressurised properly and so it boils and either pops a hose or dumps coolant into the tank.

I also did an oil change and added some oil additive to keep the engine as cool as possible.