Make sure the system is cold before attempting any maintenance

The usual cause of a cooling problem is insufficient cooling in the system. On old cars (without a see-through header tank) this is checked with the engine cold; remove the radiator cap and check by eye that the cooling elementrs are completely covered by coolant. NEVER remove the cap when the engine is hot, this will cause sudden boiling of the coolant due to the release of pressure and this is hazardous to you. On newer cars the coolant level can be checked by simply looking at the plastic header tank; the coolant should be between the Upper and Lower levels indicated. If there is insufficient coolant then top it up (make sure the engine is cold before removing the cap).

If everything is in good working order then the coolant level should not drop significantly from day to day, if it does then your cooling system may have a leak. Check the radiator, hoses and the area around the water pump for wet patches, if you suspect there is a leak then the best course of action is to take the vehicle to a qualified mechanic who will be able to advise you whether a repair is possible or you need to buy new parts. If the coolant loss is very high but you can't find a leak, get someone to rev the engine while you hold your hand in the exhaust gases; if your hand gets wet then you may have a blown head gasket or even a crack in the water jacket on the engine in which case you need to get the car to a repairer straight away.

If you don't have a leak but the cooling system is ineffective then you may have a blockage. Run the engine for a short time until it is warm (not hot). Feel around the radiator - it should be warm all over. If there is a cool spot then you may have a blockage in the radiator. Feel the top and bottom hoses; if one or both of them are cool then it is possible that your thermostat is stuck in the closed position, preventing coolant from circulating.

If you have eliminated all these possibilities and there is still a problem then it is possible that the water pump has a fault; this is harder to diagnose as most of the workings of the pump are hidden from view. If winterising has not been done properly and the engine coolant has frozen then the engine is started it is not uncommon for the shaft driving the internal impellor to break. The pulley on the outside of the engine spins normally but the impellor does not and the coolant is not circulated around the engine. If in any doubt, take the vehicle to a qualified mecahanic.