Re 80 Compression / Leak down test.

G

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John, a compression test is a means of determinimg the maximum pressure genererated in the combustion chamber as the piston moves to the top of its stroke with the inlet and exhaust valves closed.
All manufacturers will state the tolerances for this pressure in their workshop manuals.
The test can be one way of fault finding or determining the health of an engine.
Low compression pressures can be attributed to a number of things which may include - poor piston to bore sealing, burnt valves or valve seats, faulty cylinder head gasket, cracked cylinder head. Used in conjunction with other diagnostic tests - and past experience, always useful, it can help you determine faults or prevent unecessary work.
The compression tester is a pressure gauge that is has a lead on it which is fitted into the hole occupied by the injector or glow plug. The engine is turned over on the starter motor, the tester has a "tell-tale" on it that holds the max pressure generated. All cylinders are tested in turn. It is useful to see not only what the individual pressures are but also the spread of pressures.
Because of the higher pressures in a diesel engine you have to use a tester designed for a diesel engine. As I recall mine was about =A375 with the adaptors.
Some people prefer a leak down test as opposed to a compression test. Here you rotate the engine to close the valves and bring the piston to top dead centre. Remove the injector and screw the tester in. Compressed air at a known pressure is allowed in and the air supply is closed for a fixed period. Any drop in pressure is noted. Following the instructions allows faults to be determined. I have never used one of these.
Turbos - tell them what we did and what the pressure reading was.
Regards Gareth Jones. 1-HDFT Newport S.Wales.
 
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