Is this a Mk3 or 4? A friend has had a few Mk3's and they are legendary for blowing head gaskets

Yeah tell me about it

This particular Mk.3 snapped a cam shaft (!) a year ago, so I took the opportunity to swap the head for the one from my previous Supra which had a BHG. Of course the head was skimmed and rebuilt first. I had hoped to get more than a year out of a HG and in fact I suspect the problem is actually a cracked block. There is no water in the oil, the engine runs beautifully and at normal power, the only problem is overheating (seemingly due to coolant getting pressurised and forced out the header tank).

It could be a HG between a water channel and a cylinder I guess, but I believe this problem pre-dated my owning the car due to the presence of a HUGE amount of radweld or block repair compound in the cooling system. It is since I flushed out this compound that the problem has become really bad (I could only get about 20 miles to a gallon of coolant on my last trip, unless I drove in a thoroughly boring manner in which case it would stretch to about 80 miles).

For now I've chucked in some evil block repair compound but it doesn't seem to have helped much.

Rob