6 The cap that keeps its cool!

Top of cap.JPGHaving read Adrian Langmead's excellent article on diesel engine maintenance I noticed that, although the important points were explained fully when it came to the cooling system, from a maintenance point of view, the radiator cap and water pump received little mention. This is understandable because these are not "maintained" components as such, but may need to be replaced at some point because of a failure.

When carrying out job interviews for technicians I always slip a radiator cap in my pocket and, at some point, place it on the table and say, "tell me all you know about this cap". Over the years I have had some surprising (and disappointing) answers but never a full and precise one.

One young man gave me a full and accurate description of a thermostat and another told me it was a petrol cap. Job interviews seem to put people on edge and under pressure and silly things are blurted out, so these answers are not taken seriously. A little coaxing generally gets a better answer, never a full one, but a better one.

Why the radiator cap should be largely ignored by technical instructors in colleges is a mystery to me. It is a simple but very important part of the cooling system and has had a huge impact on body styling in the post second world war years, with the introduction of the pressurised cooling system together with the water pump.

So, on with the radiator cap


You may have noticed, at some time in the past while idly turning over a radiator cap in your hands, that a number is stamped in the top, anything from 3 to 15 but nowadays more commonly 10 to 15. This is the pressure in pounds at which the valve in the neck of the radiator, attached to the spring, will become unseated and compress the spring in the cap. It is, in essence, a pressure release valve, but there is more to it than that, so please read on.

Cap showing pressure relief valve.JPGWhen the cooling system gets hotter the coolant expands causing pressure within the engine, radiator and hoses. If this pressure is not released then damage may occur, such as cracking the radiator, heater matrix or splitting a hose. This damage will happen at the weakest point, so if you have 'dodgy' hoses through chafing or oil contamination, beware because you may have a failure long before the engine reaches its intended operating pressure/temperature.

Why do we have pressurised cooling systems?

Water has a higher boiling point when under pressure and we all know that water turns to steam when it boils. Escapes from a cooling system mean overheating due to lack of water, or constantly checking the water and "topping up" the level to keep the system operating efficiently. This means carrying water, stopping, waiting for the engine to cool sufficiently to be able to check and possibly adding water; hugely inconvenient I think you'll agree.

It is so much simpler to raise the boiling point by raising the pressure in the cooling system in a controlled way. Water will boil at a higher temperature under pressure by 2.5 degrees Fahrenheit per 1 pound of pressure, so a spring that is unseated at 15 pounds pressure will allow water to reach 249.5 degrees before boiling and then turning to steam.

Any escape of coolant is past the cap and down an overflow tube to the expansion tank.

You will see, if you examine a modern cooling system, that the tube goes through the cap or a port in the tank and reaches almost to the bottom. The tank is marked (usually) with a line towards the bottom and another towards the top; the end of the tube should be immersed in the coolant in the bottom of the tank to prevent air being sucked into the cooling system when the engine cools after switching off.
Cap showing vacuum relief valve.JPG
I'm sure many of you have gingerly tried to remove a radiator cap when experiencing overheating problems only to find that very hot water keeps escaping. Using towels, rags etc. you persevere, but it goes straight through the cloth and you still have to wait for the water to cool down sufficiently.

This is because you have lowered the pressure inside the cooling system and the water boils immediately, expands and escapes past the partly undone radiator cap. Attempting to remove the cap before cooling is complete, and by that I mean that pressure has been released and the temperature has dropped below boiling point, scalds many people.

So, you have completed your journey (or your work period) and have had no overheating problems and all is well with your cooling system. You park your car (or machine) for, let's say, three hours, the engine cools and all that has expanded now contracts. Where are we with the cooling system now? The water level has dropped with this contraction, but what replaces it?

There is a partial vacuum in the cooling system, and now outside air pressure could damage the radiator; look again at your radiator cap and you will see on the end of the spring a small flat valve that opens against the spring. When the coolant cools sufficiently to form a vacuum, this valve is forced open by the outside air pressure and coolant from the expansion tank is sucked back into the radiator.

Before the introduction of the water pump, engines relied on Thermo-Siphon (the less dense hot water rising above denser cold water) to circulate water around the cooling system. This was used for many years but meant very large engine water jackets and huge radiators. Just look at pictures of early cars with their large brass radiators and compare them with today's models.

The early cars always had a radiator at the front of the car to get maximum airflow through. Today's car often has the engine in the rear and the radiator, quite often, is remote from the engine altogether. So, the water pump, together with the pressurised cooling system, has had an enormous impact on body styling.

No longer those huge radiators stuck out in front, instead the lower bodylines, shorter engine bays, less weight and better forward vision, allowing lower seating positions and consequent lower centre of gravity, resulting in improved stability when cornering and on uneven ground.

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