DIY Compression Test

[jonjo]

Can an amateur discern anything useful about the quality of an engine by hand cranking via the flywheel and feeling the compression?

This is one of the tests I do when visiting a boat for sale.

I once saw a pro mechanic investigating a starting problem, on a 2 pot engine he selectively closed one decompression leaver to test each cylinder. To be honest I suspect an engine would have to be in a dire state before I would sense a problem

 

[halcyon]

Could be very engine dependant.
Our old Saab has no compression if you turn the flywheel by hand, spin with the dynostart and it has bags. The handbook list slow cranking speed as a reason for low compression.


Brian

 

[tr7v8]

To be honest compression whilst being turned over by hand is very variable due to a whole number of factors. These are type of Piston ring fitted, more modern rings are "l" shaped (Dykes) and need combustion pressure to really seal, also camshaft design makes a huge difference, in that some cam have very different initial valve lifts in order to make starting easier amongst other things.
Having said that comparing the 2 sets of compresions on say a twin should show larger anomalies up and an even better test would be to use a compression tester.

Jim
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[oldharry]

The simple answer is - no, unless you already know the engine well. If an engine that is normally difficult to turn over by hand suddenly becomes much easier, then clearly there is something amiss.

Turning an engine over you are not only driving the pistons which may or may not develop compression at such low speeds, but you are also operating various pumps, and all the valve gear some or all of which may take a considerable amount of effort so that relatively minor variations in compression may well not be noticeable. Injection pumps for example can be virtually impossible to turn by hand, as will be the camshaft as it compresses the valve springs usually 2 at a time.

 

[jonjo]

Interesting...

Would there be any merit in spinning the engine using the starter motor with one cylinder decompressed and then listening for comparable rotation speed between tests?

 

[oldharry]

Why not fire the engine up - then you will know whethere it is running abnormally or not. The noise of the starter motor, plus the blow by on the decompressed cylinder makes it very difficult to do any realistic diagnostics using such a haphazard method.

On a multi cylinder engine, a badly damaged valve will give a distinctively different 'whooshing' sound as it comes to compression on the starter, but that only tells you a valve is effectively remaining open on a cylinder. Even if the engine does run, many faults of the kind you are looking for will not show up immediately - leaky head gaskets dont always 'squeak', bearings dont rumble except on load, power loss is not evident except at speed, overheating and cooling faults will not show up, and so on.

Cranking the engine either by hand or on the starter tells you only one thing - that the engine is not seized!