Updated on 19 October, 2021
A leakdown test is used to measure the performance of internal combustion engines in a car by introducing compressed air to the engine and testing the rate at which it loses its exhaust gasses. In cars, when the pressure changes, the flow of the exhaust becomes changing as well, and the result is that gasses are leaking out. The purpose of this test is to discover the causes of these leaks and to find solutions for them. To carry out this test, a leakdown tester, also called a leak detector or an emission analyser, is used. It is placed either behind the driver's seat, in the engine compartment or under the hood.
To use a leakdown tester, first you have to locate the appropriate place for it. The most common place to install it is the inlet valve of the fuel tank, the inlet pressure must be gauged by attaching a standard air inlet pressure gauge to an external source, such as a hose or to a pressure gauge. The reason for installing the gauge in the fuel tank is to make the gauge a good indication of the leakages. If the gauge indicates an unusually high leak pressure, then it could either mean that the level of fuel in the tank is low and there is an inadequate amount of air to push through the engine or that the fuel line is blocked due to some obstruction, which requires repair.
Once you have located the appropriate location, turn the engine off. Connect the hose to the inlet valve using a screw valve or an oil-tight nut-to avoid leakage. This will start the compression test. Open the throttle to its full open position, wait for about 5 seconds and pull the trigger, if there is no visible change in the pressure at that time, then the valve is not closed.
A valve in the fuel tank is usually one of the leakdown testers on a self-adjusting air compressor; it should be next to the air filter. It is advisable to keep the pressure gauge next to the engine for quick reference. If the leakdown testers are operated through a valve on the firewall near the air filter, then the pressure will rise gradually because of airflow resistance into the engine.
One of the best leakdown testers measures cylinder pressure without opening the throttle. When the cylinder pressure changes drastically, this indicates that something has occurred in the fuel pump or the carburetor. The cylinder pressure gauge can also be used when the car is parked, as the gauge can indicate the temperature of the surrounding environment.
To use the leakdown tester, first fill the hopper with an appropriate amount of fuel. Then attach the gauge to the end of the fuel line with a 14mm plug. When the pressure in the hose changes, this is an indication of the level of fuel leakage.
Many cars come equipped with leakdown testers. However, if your car does not have these vital safety devices, then you must check the hoses and other pipes carefully. Hoses and pipes may have been damaged due to misuse, corrosion, or water leaks. Furthermore, the wear and tear of the valves may also indicate that the car needs maintenance.
Another way to find out whether your car requires a leakdown tester is to check the piston rings. If you notice a small amount of air leaking from the exhaust system, then the ring is probably defective. If you notice a large amount of air leaking from the carburetor, then the carburetor is probably clogged. On some vehicles such as diesels, the compression stroke occurs at the top of the carburetor so that both the piston rings and the carburetor are almost in the same place, thus resulting in leakages. To remedy this situation, you should take a look at the carburetor first and if it is not properly aligned, you should move on to the spark plug hole.