Typical automotive cooling system.

The cooling system should really be called a temperature control system because it is designed to bring the engine to its most efficient operating temperature as soon as possible after starting and to maintain that temperature through all operating conditions. I shall talk only about liquid cooling systems because they are in the majority.

The system relies on convection, conduction and radiation. Heat is conducted from the metal around the cylinders, valves and cylinder heads to the water jackets in the block and head.

The hot coolant is then driven out of the block and heads to the radiator where the heat is removed by convection. Air flow around the engine helps so some heat is carried away by radiation.

In theory, cooling systems require little maintenance. Of course it makes sense to check coolant level regularly and a correct antifreeze mixture must be maintained. The latter not only apart from protection against freeze-ups, also protects iron and aluminium against corrosion.

I regularly check the coolant level on my cars.

Last week I found the coolant level on the Jaguar XJ6 to be very low.

There had been no signs of overheating, but with a radiator capacity of 18 litres, it needs a lot of leakage before the temperature gauge rises dramatically.

I checked the radiator for leaks, looked carefully at the hoses and their connections and found nothing amiss. A new radiator pressure cap was fitted - just in case - but the water loss continued. No coolant showed beneath the car.

A litmus test eventually proved the head gasket was leaking. The head will have to come off and be checked for cracks and warping.

For the first time for many years, I wished the Jag was a Beetle!

Leakage of this type may not be visible externally, but the litmus test uses a chemical that changes colour when leakage is detected. The cost of such a test is very little and, if in doubt, it is well worth having such a test done.

Being keen to avoid doing the job myself, I asked at a couple of workshops how much gasket renewal and head machining would cost. The quotes came in at little less than the car is worth so I suppose I shall have to get my hands dirty again.

The last time I did this job on the same car was probably 10 years ago, so I cannot really grumble.


Normal checks of the cooling system include a look at fan belts, which should not be glazed, cracked or worn. The radiator core must be clear of obstructions, such as leaves, mud or insects. If it proves to be dirty, compressed air or water will soon rectify the problem.

Swollen or internally deteriorating hoses will restrict circulation and cause overheating. If clamps are loose air may be drawn in by the water pump and lessen the effectiveness of the system. Pay particular attention to the inlet hose to the water pump.

Relieve belt tension and grip the fan to see if there is any radial movement of the pump shaft and pulley.

Any roughness as the pump is turned suggests a worn bearing. -Star Motoring