Techniques - Compression testing
Compression testing is one of the easiest and most revealing tests that can be done on an engine.
It can be used to discover whether there is piston-ring/bore wear/damage; either exhaust or inlet valve leakage; and possibly a leaky head gasket or warped/cracked cylinder head (in combination with other tests).
Tools: Compression tester; spark plug spanner/socket; assistant (useful); Optionals: engine oil; Notepad & pencil
Note: Compression testers can be bought for £15 upwards
Basic Technique: The engine should have run recently or been spun over to free & lubricate it if stored for a period; ensure vehicle is in neutral gear; clean muck away from spark-plug bases and remove all spark plugs; if injection, disconnect electrical feed to injectors or fuel pump (optional); screw compression tester's threaded connector into spark plug hole cylinder 1; ensuring in neutral gear, crank engine on starter whilst watching gauge, stop as soon as gauge reaches a maximum reading (usually 4~5 compression strokes).
Record the gauge reading immediately against the relevant cylinder (see about check-valves later*) Repeat for all cylinders, recording the best pressure attained for each one; if in doubt repeat that cylinder...
Tip: Take care not to start the threaded connector crossed and don't overtighten it- there's usually an O-ring
against it's shoulder to make the seal!
Analysing Results: You are first of all and most importantly looking for fairly consistent readings across the cylinders, typically within 10-20 psi of each other. Next you should be judging the lowest value against the typical mean value - less than 100 psi evenly across the whole engine, might indicate long term wear and tear of the ring/bore seal. One or two cylinders significantly below any others, would indicate a serious or developing problem, depending on the comparative figures. Dependent on the engine design, figures from 125 ~ 175 psi indicate a cylinder with good ring/bore and valve sealing, although higher pressures are possible; the higher the compressions the greater any variation should be expected
Deducing the problem area when an engine shows poor compressions
Later
