When your engine feels the heat

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Overheating can cause horrendous damage to an engine, so if you do not feel competent to check it over, seek professional help.

It’s a wise driver who regularly watches the temperature gauge and investigates immediately if it climbs unusually high. A worried man told me that this fault was apparent on his elderly Ford and he had checked the coolant level in the radiator, fan belt tension and the engine oil level, and all seemed OK.

He then flushed the radiator and refilled with the correct antifreeze mix and cleaned off the radiator grill although he said there was hardly anything to clean off. The cooling fan was working too, so what could be the trouble? I took the car for a short drive and sure enough the gauge needle climbed, but the cooling fan did not cut in. Then, as the car encountered a speed hump, it cut in suddenly.

However, when I stopped the car, although the fan was still spinning, it didn’t seem quite right, in fact not fast enough to generate sufficient cooling air. Then it stopped altogether and I gave it a thump and it started again.

It didn’t need a Sherlock to work out that the fan motor was seizing and could be brought on or cut by a bumpy road surface. The owner was relieved when a new fan motor restored cool running because he had imagined a far more expensive solution and, quite frankly, the car was not worth spending a great deal of money on.

A different fault that involved the cooling system came up recently. Like oil changes and tyre pressures, the cooling system is often neglected and this may lead to expense that could easily have been avoided.

This fault involved a Waxstat device that is designed to increase engine revs during warm-up.

It works independently of the idle-speed control valve to raise or lower an air slide in the throttle body according to air requirement. The wax in it melts and changes volume as the engine warms, thus activating the slide. It is a clever but relatively simple arrangement.

When things go wrong with idle speed (normally too high), it is easy to imagine the idle-speed control valve is the culprit. This can be easily resolved by a resistance check to see if there is an open circuit in the valve’s windings.

Faults with the Waxstat itself are normally due to poor cooling system maintenance, such as using tap water in the system without coolant additives. There is a very small-diameter pipe that carries coolant through the Waxstat and this is prone to blockage in neglected systems.

Blockage causes an unusually long period of high revs that die down only when the engine has reached normal operating temperature and idle speed stabilises due to engine “heat soak”.

Granted, you will probably be able to achieve a temporary cure by disconnecting the pipe at both ends and blowing out with an air line, but the proper way to go about it is to flush the cooling system along with the air line treatment. To be on the safe side, wait a few weeks and flush the system again.

Overheating can cause horrendous damage to an engine, so if you do not feel competent to check it over, seek professional help. - Star Motoring