A couple of years ago I wrote a column on very basic engine tests and once again I have been asked to explain the difference between a compression test and a cylinder leakage test. They are both worthwhile procedures, especially on an elderly engine where cylinder sealing might be a bit suspect.
Bear in mind that the compression test measures compression pressure and the leakage test measures loss of pressure.
A compression test should be done before any sort of tune-up. The good news is that it is easy to do and compression testers are easy to come by and not expensive.
Get the engine to normal operating temperature and remove spark plugs.
It's a good idea to keep the plugs in cylinder order for diagnosis before you clean them. With the tester in place in the plug hole, and with choke and throttle plates open and coil HT lead earthed to prevent starting, crank the engine through five compression strokes. Note and record the reading and repeat the process for all cylinders.
Refer to the manual to see what ideal readings should be. If one or more cylinders show a low compression, this indicates leakage past the rings, valves or head gasket.
The next step is the wet compression test. Pour a spoonful of engine oil into the suspect cylinder(s) and repeat the compression test. If compression then reaches specifications, the leakage must be past the piston rings because the oil poured in is acting as a seal between the piston rings and cylinder bore.
If the reading stays low, then the leakage is due to a faulty head gasket or valves.
OK, now for the leakage test.
If you do it yourself you'll need a compressed-air supply complete with coupling to screw into the spark plug hole and a gauge to give the reading. All spark plugs must be removed and the cylinder under test must be set at top of its stroke (TDC). Probably it's best to get this test done by a professional.
There will be a certain amount of leakage shown on the gauge, up to 10 percent for an engine in excellent condition. An engine that is still pretty good may show up to 20 percent and is no real cause for concern. However, after 30 percent, you should begin to worry.
One of the advantages of this test over the compression test is that escaping air will tell you exactly where the fault lies. For example, a hiss from the exhaust suggests a leaking exhaust valve. A leaky inlet valve would show by expelling air from the carburettor or throttle body.
If air escapes from the oil filler cap or positive crankcase ventilation (PCV) valve, you can bet that there is wear in the piston ring and bore area.
But what about an engine that shows low on the compression test but has very little leakage? If this happens, check the valve timing. The belt could be a tooth out. - Star Motoring