Many people worry about overheating when infact they have not actually measured the engines temperature. It is common to run on, on a car using unleaded fuel, as it burns hotter. Running on on leaded fuel cars is either:

But it could also be an incorrect thermostat.

If the temp gauge reads higher than usual, other checks are needed, but first check the gauge. They are notoriously inaccurate. They are cheap things anyway. One substituted from an autojumble may even read too cool. Use a thermometer in the rad when you think its too hot. Take care removing the rad cap. Your car should have a four bladed fan, and the matrix clear of dead flies and grass. How do I know it is overheating? Well, it boils. You will be constantly losing water out of the overflow pipe. The engine will be a pig to restart after a long run. The heater will seem very hot. Exhaust valves will burn out. The gauge will read off the face as it only covers from about 80 to 100 deg (Celcius) in its full deflection. DO NOT trust your hands to tell you its hot, as the radiator top will be far too hot for you to judge.

How do I cure this? First check the fan belt tension. The check you have the correct pressure radiator cap. Remove the hoses and rad-flush the block, radiator, and heater. It is best to remove the radiator to do this. Remove the water pump and clean up the impellar or fit a new one of the correct size (ie not a 1489 to a 1622.) Remove the various plugs and taps in the block, and clean out the holes, using a drill bit in your hand. Lots of muck will come out here. If it still overheats after this, plus a servicing, you've got some work to do!

Right, take off all the hoses, the water pump, the rad, and flush them out. Now remove al the dynamos, etc, punch out the core plugs, and flush the block. You can get the read one out by dropping the gearbox cross member six inches, and going in over the bell housing. Fit new core plugs, and have a hour core element fitted to the radiator by a specialist.