Techniques - Compression testing

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What, When and why?

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).

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.

On VW WBX flat-fours, No. 1 is said to run hottest and thus usually have lowest compressions.

Compression pressures: throttle fully open; all plugs removed (earth central HT lead); oil temp > 30°C;

Compression pressures - Bar (psi)
Engine Min Typical Max diff
WBX 1.9 8 (116) 10 (147)~ 13 (189) 2.5 (37)*
WBX 2.1 8 (116) 10 (147)~ 13 (189) 2.5 (37)*
Air Cooled 2.0 5 (73) 6 (87) ~ 9 (131) 3 (44)
Diesel 1.6 NA 11 28 (406) ~ 34 (493) 5 (73)

* Not quoted, so estimated

Interpreting poor compressions

See forum discussion below

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 400~550 psi.

Method is basically the same, but due to the cost its usual to ask a garage to do these tests

Engine oil temp > 30°C; disconnect fuel solenoid & return bleed hoses; remove injectors; replace olf heat shield in inhector hole (to checkit); screw adapter of tester into injector hole; neutral gear;

Do tests; re-check any that are low with new heat-shield

Re-install injectors with all new heat-shields.

Forum extract - walk thru


Take out all the plugs, disconnect the HT lead from dizzy and earth it to chassis (croc clips and wire or jump leads sort of arrangement but keep everything loose away from fan belt), turn over usually 5/6 turns will give you constant reading, but note the first instant reading as well as final stabilised one. Do twice on each cylinder to check that you are getting a consistant result. The compression tester will have a reset button.

This was mine

  1. 1 7.5 bar/9.5 bar
  2. 2 9bar/10.5 bar
  3. 3 8 bar/10 bar
  4. 4 9bar/11 bar

Within spec, a bit worn, #1 always lowest as runs hottest, as furthest from water pump, #4 best, next to pump.

Dubstar: You can also do a wet test.

On mine the instructions were to put a cap (NB. cap, not cup!) full of engine oil down the cylinder bore through the spark plug hole and then do the compression test. One cylinder at a time!

So, do the dry test first and then the wet test. It's supposed to give an indication of how worn the bores are, depending on the difference between the 2 readings. On my old CU the differences on 3 cylinders were ok, but on the 4th the compression readings wet and dry were both the same, and low, which confirmed what I thought and that is it was down on one cylinder. The readings on my Aircooled CU were:

138, 140, 121 & 110 dry

152, 152, 160 & 110 wet,

+ 10%, 9%, 32% & 0% from dry to wet

going from left rear to left front, right front and right rear.

HarryMann: Then that's probably a burrnt valve problem on right rear, as they didn't change with oil..

With a ring problem on right front (121 > 160)

Morganman: Smokes on overrun, is this valves/guides or bore wear

1= 9.9, 10.1 (+2%)

2= 10.5, 12.0 (+14%)

3= 10.2, 11.2 (+10%)

4= 10.4, 11.9 (+14%)

AV. of 150 psi (10.25 bar), yes more like it

PS. As Aidan says, go round engine once, and then go around again and check they are the same, any doubt repeat that cylinder.

Write them down as you go...

  • and make notes*

Then do the oil test if you need to... (low pressure on some or all cylinders)

Always remove all plugs before starting so that cranking speed will be consistent and battery not become drained, speed can make a difference to a bad cylinder..

  • A good cylinder will also pump up almost immediately (within 2 or 3 compressions)... so 'watch' the gauge as you crank the engine (use a helper or remote solenoid switch)*

Leak-down test

Then there's the leak-down test... checking rate at which they leak back from compression, comparing them, and listening in exhaust or inlet for valve seat leaks...