Techniques - Compression testing

Compression testing is one of the easiest and most revealing tests that can be done on an engine.

Method

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).

It is also used to benchmark an engine when first acquired, or tested before any significant changes for the same reason

Tools: Compression tester; spark plug spanner/socket; assistant (useful); Optionals: engine oil; Notepad & pencil

Note: Compression testers for petrol engines 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 one; 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

Remove the connector, pop the one-way valve to zero the gauge

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

On an in-line 4-cyl engine, often No.3 (3rd from the front, pulley) runs hotetst and is often the first to succumb to cylinder or burnt-valve trouble. With Subaru boxers, the front left overheats easiest.

Deducing the problem area when an engine shows poor compressions

Later

Diesel engine compression testing

This requires a different gauge, that cost quite a bit more than petrol ones. These usually screw into the fuel-injector hole and go to a lot higher pressures, circa 450~550 psi. Method is basically the same, but due to the cost its usual to ask a garage to do these tests