Compression testing, it's not that good or honest.

red reading

Active Member
Compression testing,

where to start, well the first thing to realise is that it is only really part of a process for diagnosing a problem, things listed below will change compression readings:-

1. Carbon in the combustion chamber will give you a high reading,
2. Too much oil on the bores will give you a high reading,
3. Cams and their timing will change your readings
4. A engine not fully warmed up will give you false readings,
5. not having the throttle wide open will give you false readings,
6. Valve clearances that are wrong will,
7. altitude will vary the reading due to atmospheric pressure changing.
8. Lazy starter motors give low readings.
9. Battery condition will affect the readings.
10. Worn big end bearings will change readings
11. Massive variations in machining of internal components in mass production can change readings.

so in summary the only thing it is only good for is a very rough indication of an engines health a very rough guesstimate , there are way to many variables to even contemplate taking any readings as being true. Now the correct way of checking the engine is by using a leak down tester as this can give you an indication of not only the cylinders condition on top dead centre but also at any position in the bore, also because you are not relying on any parts moving by turning the engine over your getting rid of variables that give false results.

because you are pressurising the bore you can hear air leaks from any worn components such as leaky valve seats or worn rings, this is the correct test for checking an engine for its true health.

now if you have an engine issue after using the leak down tester you can eliminate the piston ,bore and valves if the results are good.

for even better ability to see inside your engine you can now buy bore scopes on line for £70 and up wards, with a bore scope you can see any issues from detonation,ring land failure, headgasket issues etc.
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Staff member
I think the bore scope is a great idea; nothing better than actually taking a look at something to see what's going on. - I can't say I'd know what I was looking at, but I figure I could recognise obvious damage.