During the dark days of engine building, car makers used to depend on a compression tester to check whether they had a weak cylinder, by comparing the cylinder's cranking pressures against each other. But this was a crude and inaccurate method to determine how well the engine was sealed. Hence, to overcome these drawbacks, nowadays a leak down test is used to evaluate how well each cylinder performs in retaining pressure, irrespective of its counterpart.
The tester determines the condition of engines by inserting compressed air into the cylinder and evaluating the rate at which it leaks out. There are primarily three main points from where the cylinder pressure may escape: past the rings, the intake valve, and the exhaust valve.
The testing is done when the engine is not running, with the tested cylinder at top dead center (TDC). It can also be done at other points in the compression and power stroke. Through the spark plug hole, pressure is fed into a cylinder and the flow, which indicates leakage, is then calculated. The leakage is measured in arbitrary percentages, which do not refer to any literal quantity or dimension. The implication of these readings is relative to other tests. Leak down readings approximately up to 20% are acceptable, but more than that requires a repair.
How to Perform a Cylinder Leakage Down Test
There are two different tools used for conducting the test: single and dual gauge units. The single gauge unit, though easy to operate, requires special attention at the inlet pressure, as too high or too low a pressure can give erroneous results. The dual gauge unit also requires a particular range of inlet pressure, which is measured by the first gauge, while the second gauge measures the percentage of cylinder leakage.
Before pumping the air into the engine, make sure the engine is warm and all spark plugs are in the engine, except for the cylinder you are testing. Jerk the plug and turn the engine over so that the piston is at TDC. Put the air-fitting adapter into the spark plug hole, and don't forget to check that the gauge reads zero before you start. You may also require a breaker bar, or ratchet and socket for the crank nut.
The main concept is to put air into the cylinder and move the piston around the TDC so that the pressure seals the rings to the bore. As the rod journal develops leverage on the crank on either side of TDC, the cylinder pressure along with the leverage pushes the piston down the bore. Now measure the leak down percentage and test the remaining cylinders in the same manner.
A cylinder leak down tester can be considered as a diagnostic tool to identify cylinder pressure problems. Don't forget that no cylinder seals perfectly, especially if the engine is supercharged or turbocharged, where the top and second rings are set with wide ring endgaps. So, if the leakage numbers are around 10 percent, with a margin of 4 to 5 percent, it is not at all a cause for alarm or worry.