19.3.
Cylinder Leakage Test
Another test that measures how well the cylinders seal is the cylinder leakage test. This test also identifies the parts that are worn or defective, causing the leakage. Although the cylinder leakage tester (Fig. 19.14) performs about the same thing as the compression tester, but it does it in a different way. During the leakage test, the engine does not have to be cranked. Instead of measuring the air pressure formed on the compression stroke, the leakage tester applies air into the cylinder and measures the amount of air that leaks out the cylinder past the valves, rings, or head gasket. With the piston at TDC in the compression stroke, both valves are closed. This should allow very little air to escape when air is introduced by the leakage tester. It is normal for some air to leak past the rings, but the amount should be small. The cylinder leakage tester can precisely locate the points of leakage. These points of excessive leakage are found by feeling or listening to the air escaping.
Fig. 19.14. Cylinder leakage tester.
The cylinder leiage tester compares the amount of air pressure applied by the tester to the cylinder to the amount that the cylinder is able to hold. An outside air supply is needed to use this tester. Usually, automobile shops are equipped with an air supply. If the air supply has 690 kPa pressures and the cylinder is able to hold 550 kPa, the cylinder allows 20% of the air to escape. The escaping air can be felt or heard from the carburettor or throttle body if it is escaping past the intake valve; from the tailpipe or exhaust pipe if it is escaping from the exhaust
valve; from the oil filler tube if it is escaping by the piston rings; and from the radiator if it is escaping from the head gasket or a cracked cylinder head or block.
To use the tester, warm up the engine, then remove all the spark plugs. Now remove the air cleaner assembly, oil filler cap or dipstick and radiator cap, and block the throttle to a full wide-open position. Check the fluid in the radiator and fill it to the proper level.
The tester is fitted with a hose that threads into the spark plug hole and fastens to the tester. Install the appropriate size hose into the number 1 cylinder. Most testers are also equipped with a whistle to help locate TDC. If a whistle is available, connect it to the tester end of the hose. Crank the engine until the whistle sounds, which means that the piston is moving towards TDC on the compression stroke. If no whistle is available, put the thumb over the end of the hose to feel the pressure building up. Continue to rotate the engine until the timing marks on the crankshaft align at TDC. At this point the engine is at TDC on the compression stroke and the valves are closed. Disconnect the whistle from the hose the connect the hose to the tester.
Now, connect the shop air supply to the tester. The gauge on the tester should show a reading. This reading indicates the amount of air that is leaking out of the cylinder. Listen at the radiator, oil filler, carburettor, and tailpipe for the escaping air. Also check if air is leaking from cylinder next to the one being tested. If air is escaping from an adjoining cylinder, the head gasket leaks between cylinders.
Test the other cylinders in the same way. Make sure that each cylinder is at TDC of the compression stroke before applying air into the cylinder. One can use the distributor’s rotor and its alignment with the terminals of the distributor cap to locate TDC of the different cylinders. If the piston is not at TDC, the air applied to the cylinder may push the piston down, and this causes the valves to open as the crankshaft turns. To measure the amount of leakage in a cylinder accurately, the valves must be closed. If high amounts of leakage are recorded, disconnect the air supply and attempt to reset TDC again to repeat the test.
Gauge readings of less than 20% are normal unless there is a noticeable amount of air leaking through the carburettor, exhaust, oil filler, radiator, or adjacent cylinder. Any reading over 20% is on the higher side and the location of the air leakage identifies the problem. Obviously, if higher is the leakage, the greater is the problem.
The cylinder leakage tester has an advantage over the compression tester because it provides precise identification of the problem area. Carbon build-up also does not influence the readings. Therefore, this test tends to provide a truer picture of the mechanical condition of the engine. The biggest disadvantage of this test is that it requires more time to conduct than the compression test.
