When a simple compression test reveals compression loss no matter the percentage, it will be necessary to conduct cylinder leakage test immediately.
The leakage test can be carried out to know the actual percentage of the compression loss and to help locate the actual source of the leakage. The result of the test will guide the engineer on how to make repairs in the engine.
To carry out the test, the cylinder must be ensured to be at TDC on its compression stroke to ensure that the valves of the cylinder are closed before the introduction of air into it. After that, a cylinder leakage tester will be used to apply compressed air to the cylinder through the spark plug hole.
A threaded adapter on the end of the air pressure hose screws into the spark plug hole. The air source is usually from a compressed air system in the workshop, during the process; a pressure regulator in the tester will control the pressure loss from the cylinder and when that happens, a dial gauge will register the percentage of the air pressure lost from the cylinder when compressed air is applied.
A properly calibrated dial gauge reading from 0% to 100% will be used for the percentage loss of air in the cylinder during the compression stroke. The result of the reading will give directions on how to trace the root causes of the air leakages in the engine cylinder. When the gauge reads 100%, it means the cylinder wouldn’t sustain any pressure during compression, a condition capable of making the engine unable to start. While indication of 0% on the gauge shows that there is no air loss in the cylinder during compression.
To locate where the leakage is coming during the compression, listen closely to the cylinder and use your skin to feel the engine body to take note of any possible air pushing on the skin.
The openings in the cylinder such as the valves and plug openings should be the first places to check for possible leakage. Dropping a liquid can reveal the part through its babblings when dropped around the openings of the valves and plug.
During the process, if air is felt or found leaving the throttle plate assembly; a leaking intake valve has been indicated, in other words; the intake valve is leaking which may be the actual source of the drop in the compression of the engine cylinder. If air can be felt leaving the hole of the cylinder next to the one being tested, then a bad exhaust valve is responsible.
The problem of air leakage coming from a bad head gasket or cylinder will be shown is air is felt or heard coming from the spark plug hole of the cylinder next to the being tested. Air leaving the radiator indicates a faulty head gasket or cracked block or head. If the piston rings are bad, air will be heard leaving the valve cover’s breather cap or the oil dipstick tube.
Normally, most engines do have some level of air leakage during their compression strokes including newly made engines, no engine can guarantee 0% air leakage, but from 0% to 20%5 had been specified as the most acceptable leakage rate in any engine, beyond that the effect will become visible to the overall engine performance.
Also, the age of any engine will determine the possible air leaking ratio since low air leakage means higher power stroke of the engine leading to a higher power output of the engine. Wearing and tearing creates more clearance in engines as days go by making the air leaking percentage to unnoticeably continue to rise until its effects on the engine performance become visible.
