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Overheating 101

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What you need to know to keep your cool.

We’ve all seen it and many of us have been there – classic car pulled over, hood up, steam pouring out. You can almost guarantee it every time there’s a line for a summer event. But it shouldn’t be so: classics made before the 1980s should be less likely to overheat than many more modern cars.

By the 1980s, electric fans had taken over. A properly installed mechanical fan cannot fail (unless the fan belt breaks, in which case you have warning and can replace it), whereas electric fans depend on good electrical connections, a functional thermostatic switch and sound fuses.

In a hot, dirty engine bay, those fail and that’s why you see modern carss on road shoulders whenever gridlock develops – the electric fan simply doesn’t come on to cool the radiator. And that’s one good reason not to rush out and convert your classic to an electric fan. The other is that an electric fan is not necessarily as effective. Ironically, the best argument for fitting an electric fan is if you are going to a very cold country, where a mechanical fan pulling super-cold air through the radiator first thing before the thermostat opens could freeze it, even with anti-freeze.

Introduced in the 1960s to reduce the noise and wasted energy of mechanical fans at high revs, the viscous coupling was a great idea. However, they deteriorate with age. With the engine off, try turning your fan – if it spins freely, the viscous coupling has failed: replace it as it will not cool your engine efficiently. It should turn, but with resistance.

The biggest cause by far of overheating in old cars is a silted-up radiator. There’s a common misconception that you can fix this by back-flushing with a garden hose. This may remove loose deposits but if you’ve seen how limescale builds up in a kettle or central heating pipes, you will know that the real problem is the layer of hard lime that builds up gradually on the surfaces. It’s a good insulator and, long before it actually blocks the 
pipes, it reduces flow and dramatically reduces the heat-exchanging capability of the radiator.

Just because water flows through it, that doesn’t mean the radiator is fine: in a hard water area, radiators last less than eight years and only if you use deionised water and anti-freeze exclusively will you get more than 10 useful years. If your car overheats and the timing, mixture and cooling fan are correct, replace the radiator – you won’t regret it.

Before you take out the old radiator, use a flushing compound to flush as much as possible out of the water passages in the engine and heater. With luck, they’ll get deposited in the old radiator before you chuck it out.

Other cooling problems can be improved by replacing the thermostat (some fail closed, some stick partly open, restricting water flow). Never run a road car without a thermostat, because you will accelerate engine wear and increase fuel consumption, as it takes longer to warm up and will wreak havoc with a temperature-controlled automatic choke.

A worn pressure cap that no longer seals properly will allow coolant to leak out and lead to early boiling; it will also encourage air locks. Look out for these if your cooling system regularly loses fluid without boiling; some cars have bleed valves at high spots to let air out, on others you may have to loosen a hose until all air escapes. Jack up the car if necessary so that the filler cap is higher than the rest of the system when filling.

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