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If you see a fellow classic owner by the roadside this summer, all hot and bothered, then the chances are is that the beloved car (and driver) has overheated. Sadly, it’s a common enough occurrence, yet this needn’t be because they didn’t boil over when new did they (Stags excepted, of course!)?

Every classic owner should always keep an eagle eye on the temperature gauge when out and about although don’t get paranoid about it because if the cooling system is in tip-top shape then all should be well – if you follow this advice.

Routine checks

Start with checking the coolant level before a long journey – can you spot signs of coolant leaks, easily identified by traces of anti-freeze? Check the hoses for ageing and bulging past their clips (which should also be in good order and preferably secured by proper ‘Jubilee’ types). However, simply because a hose looks alright, it doesn’t mean that it hasn’t collapsed internally, so remove and check it if in any doubt.

Now, check the radiator cap including its spring and rubber seal – replace if the cap looks past it but do check it’s the right poundage as many engines had this revised over their production runs. As it affects the system’s efficiency as well as the water pump’s performance, are you absolutely certain that the correct poundage one is fitted?

With the engine up to temp, remove said cap (extremely carefully we must stress) to check that the water is circulating, indicating that the thermostat and water pump are doing their job giving a vigorous flow. If this isn’t the case (and it may also mean a blockage) then consider replacing both to play safe.

Thermostats aren’t just open and shut cases; they are constantly operating, and so they wear out. A new ’stat can work wonders, and you can even change the operating temperature by fitting a ‘winter’ one, which opens at a higher temperature, and so makes the engine run hotter, if desired. Don’t be clever and be tempted to run without a ’stat thinking that it will halt any cooling woes since, on some engines, it forms part of the circulation route and the lack of the ’stat upsets the flow, leading to even greater overheating – even engine seizure if you’re not careful!

A good periodic flush through the internals with a hose, in conjunction with one of the numerous de-scaling additives widely available, is sensible if it hasn’t been done for many years or if you have recently bought the classic but follow the product’s specific instructions for them to work properly.

There are two flushing methods, a normal flush and a reverse one, which as the name suggests goes against the normal coolant flow. Some mechanics are opposed to this as any gunge can be forced back and clog the system elsewhere. First decide if you aregoing to use a special descaling additive; follow instructions fully. Flushing means removing the thermostat and replacing the housing and turning the heater to its ‘hot’ position.

Disconnect the top hose (if the rad is still in situ – plug the stub up but with rad cap removed). Insert a hose in the top of the radiator and fully turn on the hose – some advocate running the engine at the same time so pump encourages speedier flow. A reverse flush means you fill via the thermostat housing – consult an expert if in doubt.

Finally, remove the radiator to flush out the cooling matrix – both sides. It’s amazing how much muck accumulates over the years (decades!), especially on BMC side-mounted types (Minis etc). Even though it looks ok and isn’t leaking perhaps the radiator is simply past it? Some experts suggest replacing them after a decade and if you wonder why, check the weight of your car’s rad against a new one. We bet the former will be significantly heavier due to the build up of internal limescale and other nasties…

Why use water?

There are some alternatives to the traditional antifreeze-based engine coolants, such as Evans Waterless Coolant. This doesn’t operate under pressure, unlike antifreeze-based engine coolant, so there’s less strain on the hoses, connections and the radiator. To use it, all traces of coolant and water need to be removed by filling the engine’s cooling system with a prep fluid, before draining it and then adding Evans Waterless coolant and bleeding the system. The only disadvantage with this special coolant, apart from its price of around £100, is that it cannot be topped up with water, so it’s wise to carry a container with some of the product, just in case, and its seeping characteristics means the system has to be fluid tight.

Hot running

An air lock in the cooling system is the automotive equivalent of trapped wind – and is just as painful! Likewise, it’s usually caused by gulping things down, in this case large quantities of water, say after replacing a hose? Air becomes clogged and doesn’t allow proper circulation thus hot spots occur, causing the engine to sporadically overheat.

To rid the system of an airlock depends on its severity and location. In many instances, simply running the engine up to temp with the rad cap removed and regularly squeezing the top and bottom hoses should do the trick, otherwise it means disconnecting a heater return hose, running the engine until water (take care it’s not hot) stops flowing out before resuming evenly, signifying the air has been expelled.

Before any of this though, you must ensure that the car’s heater is fully on and the lever in the ‘hot’ position. This is how air locks can form here and if in doubt check that the cables and water valve are operating properly, otherwise you’ll never get rid of the problem. Similarly, before any major refills, set the heater to the ‘hot’ position.

A similar situation, together with water loss can be caused by a failing head gasket. Familiar signs include an exhaust pipe doing a fair impression of a dripping hose and ‘combustion’ smell in the expansion/header tank. If in doubt have a specialist or garage carry out either a traditional compression test or a chemical check of the coolant – the modern method for around £25.

A nagging temperature needle can be put down to poor engine tuning. An over-weak fuel mixture and/or incorrect ignition timing can cause an engine to run hotter than normal.


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