Martynn Randall is technical editor at Haynes and has been with us for 27 years. He's written more than 60 Haynes publications and has owned more than 85 cars and 60 motorbikes... so far!
Coolant leaks are frustrating, no question. Having to top up the expansion tank once a week is deeply annoying if you can't find an obvious source of the leak.
Often, a leak will only be apparent when the engine is hot; as the temperature increases, the cooling system pressurises, the coolant finds the weakest point in the circuit and tries to escape.
The best way to find this kind of leak is by using a pressure tester.
How do I use a pressure tester?
This inexpensive piece of kit connects to the filler neck on the cooling system expansion tank and, after a few pumps of the operating handle, pressurises the system, mimicking the conditions of a hot engine without the inconvenience of you scalding coolant.
In the first instance, pressurising the system will determine if there is a leak or not. The tester comprises a gauge which measures the pressure in the system. If the reading on the gauge doesn’t drop within 10 minutes or so, there's no leak in the circuit, but if the reading falls an external leak can be identified by a fine spray or dribble of coolant.
An internal leak is harder to find, but if the pressure consistently drops and no coolant appears, the leak is almost certain to be internal.
It’s important not to over-pressurise the coolant circuit with the tester. The expansion tank or radiator pressure cap has a built-in valve, designed to open when the pressure in the circuit reaches a certain value.
The cap normally has this value given on the top face, and is usually around 1.1 bar. There’s no point in pressurising the coolant circuit beyond this rating because this would stress the circuit components unnecessarily.
Depending on the design of the cap, the pressure testing equipment can also check the expansion tank/radiator pressure cap. The cap is simply attached to the tester, then pumped up until the cap valve opens. If the cap is in good condition, the valve should not open until the pressure gauge reading matches the rating of the cap.
It’s common for the valve spring to weaken with age, in which case the valve opens too soon, losing coolant when the engine is at normal operating temperature.