How to Test Radiator Cap

January 27, 2012

How to test a radiator cap is something that should be known by motorists, although it is usually done when vehicles are serviced. Radiator caps are a critical part of the cooling system of a motor vehicle as they seal the opening used to add water to the radiator. The liquid in the radiator assists in preventing the vehicle's engine temperature from getting too hot or too cold. When the engine is running, the temperature of the liquid rises and the liquid starts to expand, causing an increase in pressure in the radiator. Most radiator caps are designed to not only provide a watertight seal but also to act as a valve to release the pressure. If a radiator cap is malfunctioning, the result can be internal damage to the engine. Cooling liquid will seep out of the radiator and, as there will be less liquid in the radiator, this could result in overheating problems.

Vehicle manufacturers recommend that motorists have their radiator caps tested at regular intervals.

Option 1 – Tested by a Mechanic

If you want to know how to pressure test the radiator cap, watch the motor mechanic next time you have your car checked, as testing equipment can be interesting. The equipment consists of a pump and some adapters that look like the filler necks of the radiator. After the radiator cap has been installed, the hand pump will be connected and air pressure will start to pump into the sealed adapter. While this is done, the pressure gauge will be watched to test if the radiator cap is leaking. The pound measure is how much force can be applied before the radiator cap relieves the pressure.

Option 2 – Test it Yourself

It is possible to pressure the radiator cap yourself using a radiator pressure tester. Try borrowing one from a neighbor as it may be a bit of a luxury to buy one.

Open the hood to access the radiator which will be situated in the front of the engine. The radiator cap must be twisted counter-clockwise to remove it. Remember to push it down before you start to twist it. The next step is to screw the cap adapter to the end of the pressure tester until it stops turning. If you are not sure what it is, then look for the label in the packaging or ask the person who has lent it to you. Place the radiator cap in the other end and tighten it on as you would to the radiator.

Pump the tester with the pump handle at the other end of the tester and watch the gauge to see if it holds the pressure. Ideally the gauge should reach at least 15 pounds.

Faulty Cap

If the pressure cap is not functioning correctly, then the gauge will start to drop. Unscrew the pressure cap from the tester and clean any sediment or debris off the pressure cap. Re-test the cap at this point to ensure the leak was not due to blockage. If the pressure cap fails this test then it is no longer of any use to the coolant system and should be replaced.