I have a '94 316 now as my first car. I was advised not to buy it cos it's general knowledge that it had been thrashed by the previous owner but I did. Anyway, its been sitting up now for 3 months cos it has a major overheating problem. Temp is fine for about 15mins of idle but after that it rockets up and the rad cap has gone bang the first time it happened. I've replaced the thermostat and filled it from the top hose down to prevent airlocks (got me the first time!). Before i attempt to change the water pump has anyone any idea of what it else it could be and the steps i should take to diagnosing the problem? The car is fairly modified so 90% of garages won't touch it!

You should really check the water pump, sometimes the impellers break and/or come loose and don't pump. Another thing would be that the radiator could have blockages in the passages that won't allow proper cooling.

If it is that highly modified you shold really overhaul the cooling system. Get an aluminum radiator, new water pump, all new hoses, and a full coolant flush.

One thing you might want to check is your head gasket.

Could be a blown head gasket.

a mate's goin to bring down a device for checking if the head gasket is gone, then i reckon it'll have to be a new water pump. Cheers!