There are two very good tests that may be used together or separately to show how well the engine’s cylinders are sealing. The first, with which most of us are familiar, is the compression test.
Briefly, to carry out this test you first remove the spark plugs.
With throttle open and choke off, hold the tester firmly in position and crank engine via the starter motor for at least four compression strokes.
Note the readings of each cylinder. On a car chosen at random, as an example the standard compression pressure of 12.7 bar the minimum permissible compression pressure would be 9.8 bar and maximum permissible difference between cylinders 0.97 bar. Check the workshop manual for figures for your car.
When a cylinder shows a low reading, pour about a tablespoon of engine oil in through the spark plug hole and recheck.
If the reading improves, the rings are worn; if the reading does not change, suspect badly seating or sticking valves. When two adjacent cylinders show low readings, look for a gasket leak between them.
LEAK-DOWN TEST
The other test, which is not quite so well known to the home mechanic, is the leak-down test.
This also shows how well the cylinders are sealing, but measures pressure loss rather than pressure.
Many garages will do this test for you at reasonable cost.
To begin, the spark plugs are removed then the engine turned so that the piston in the cylinder to be tested is at top dead centre (TDC) with both valves closed.
Then a coupling with a gauge attached is screwed into the cylinder and compressed air is fed in.
An engine in top class condition might show between five and 10 percent leakage. Even up to 20 percent is still pretty good, but if it shows 30 percent or more, that is bad news.
One advantage to this test over a compression test is that in the event of leakage/pressure loss the escaping air will give a hissing sound to enable you to locate the trouble spot. For example, a hiss from the exhaust indicates a leaking exhaust valve. Air emitting from the carburettor or throttle body means leakage from the inlet valve.
Neither is too serious and regrinding the valves and seats should put things right. More serious is the dreaded hiss coming from the PCV valve or oil filler cap meaning ring/bore wear.
MANY POSSIBLE CULPRITS
The leak-down and compression tests may be used together. If you find a cylinder with low compression, but very little leakage, there are many possible culprits such as a broken valve spring, badly worn cam lobe, bent pushrod or something amiss with a hydraulic tappet.
Low compression on all cylinders, but with minimal leakage, could mean incorrect valve timing. Check to see if the timing belt or chain has jumped a tooth.
There are various possible causes if a cylinder misfires.
It could be an ignition problem; the cure involving no more than renewing HT leads and/or spark plugs. It could also be due to an injector playing up.
If the latter, it is best to have all the injectors professionally checked and cleaned or renewed as necessary. -Star Motoring