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by Handyman
Bench wise.coo/it (32)
IN THIS, the final article in my "Cool it" series, I am going to deal with the radiator cap because it does play a most important part on the modern vehicle. Whether mounted on the radiator core top tank or on the overflow tank, its purpose is to prevent overflow loss during normal operation and enable the engine to be operated at a higher coolant temperature than otherwise.
The operating parts consist of a spring-loaded valve in the cap which closes the outlet to the overflow pipe of the radiator to atmosphere, or to the overflow tank, and effectively seals the system. This causes pressure to build up, raising the boiling point of the coolant without loss. When the pressure in the system rises to the rated opening pressure of the valve, the valve is forced off its seat, permitting steam and coolant to escape via the overflow until the pressure drops and the valve can close.
The second valve in the cap is vacuum operated and open as the system cools down, allowing air to enter the system or coolant to return from the overflow tank; this valve also prevents any collapse of hose-pipe between engine and radiator.
Airtight cooling Correct operation of the cap depends upon an airtight cooling system as well as tightly seated pressure and vacuum valves in the cap. The fit between the cap, the filler neck gasket and seating shoulder on the neck must also be pressure tight. To test the air tightness of the cooling system, attach a rubber tube securely to the overflow pipe of the radiator and block open the pressure valve; with the cooling system cold, suck on the tube, apply the tongue to the tube end and if the tongue adheres to the tube opening the system can be considered reasonably tight.
Where there is an air leakage, this can often be found by blowing tobacco smoke into the tube and watching to spot the point from which it escapes. These tests will reveal an air leak in the valve cap seat and any other leaks above the coolant level; any amount of coolant found in the overflow tank is an indication that the coolant is not being returned to the radiator — certain evidence of air leakage. Should a vacuum valve fail to open when the system cools down, this fault can be spotted quite quickly by inspecting the hose-pipes as signs of flattening or collapse will be in evidence. If after washing out and cleaning the valve it does not come off its seat under normal finger pressure, the cap should be replaced.
I mentioned the temperature gauge on the instrument panel in an earlier article, but have not considered the possibility of it failing, or giving incorrect information. However, these possibilities should not be overlooked in the event of any over-heating that is not straightforward. This instrument is some distance away from the actual thermal unit that reacts to temperature changes. The gauge is usually scaled in degrees Fahrenheit, and as a rule does not give a reading until the thermal unit is experiencing approximately 100deg F.
False reading If various other tests indicate a false reading or no reading at all from the gauge, then it should be tested against a known, reliable gauge placed in the coolant. For a really reliable test, the thermal unit should be removed from the engine and placed in a bucket of water that has been brought up to 120deg F, the test thermometer placed in the water and its reading and that of the dashboard instrument compared. Take several readings with the water in the bucket at various temperatures — any variation between the two instruments should not exceed 5deg F.
This test applies equally to electric temperature gauges as to the liquid expansion type, although a little extra care is needed where the gauge is simply marked cold, normal and hot, etc. In practice it is not now considered important that the driver should read off figures in degrees F, only that he should recognize when the instrument is recording above or below his regular normal needle position.