Diesels are naturally more noisy than petrol engines. The loud combustion noise of a diesel can drown some minor rattles but it should be possible to detect anything really serious.
Fortunately, the noises produced by piston/bore wear and knocking big-end bearings are fairly easy to confirm by smoke emissions and low oil pressure, respectively. These are the two conditions that need treatment on a fairly urgent basis.
Conversely, noises from lesser problems, like timing chains, tappets and even fuel injectors, are amplified by the covers against which they are knocking, with the result that they can sound almost terminal. So my advice is not to put too much reliance in noise levels but to look for confirmation by other means.
There's one exception: with very low engine power, a loud clanking sound coming from the engine at the expected rate of one cylinder firing. The sound seems to be coming from the top of the engine and is more obvious with the filler cap off. The likely cause is a broken valve spring.
“Proper” compression tests
Unlike gasoline engines, for which compression testing equipment can be bought cheaply at your local car parts shop, the equipment for diesel compression testing is specialised and expensive.
Firstly, the fittings for injectors and/or glow plugs are not standard and almost every major manufacturer uses a different type. Thus a custom built, and inevitably more expensive, connector is needed for each engine type.
Secondly, the pressure involved can be a lot higher and the test equipment is built to a superior and safer standard. Compression testing is better carried out by a professional, but only after the owner has made some preliminary checks that will give almost the same information.
If you are able to borrow or hire the correct tester, compression tests should be taken when the engine is at operating temperature. Remove a glow plug or injector, screw in an adapter and connect the pressure gauge.
Measured pressure for engines varies considerably, perhaps between 200 and 500 p.s.i., dependent upon design and condition. It is far more important that the measured pressure is similar for all of the cylinders than that the value is a certain figure. However, a very low figure would indicate that all cylinders were worn.
Substitute compression tests
You can make a good subjective judgement of the compression of an engine by turning it over manually. You will need a socket that fits the crankshaft pulley nut and a tommy bar that fits the square drive.
These nuts are sometimes larger than is provided in the average socket set, in which case buy one, but as it will not be taking any significant load the cheapest sold by your neighbourhood car parts shop will be quite good enough.
Bukh and older Volvo engines may not have a single nut but you should be able to turn the flywheel instead, after padding the starter ring with a rag.
Make your initial test with the engine completely cold. Fit the socket and tommy bar to the pulley nut and turn in the direction of rotation. The effort required to rotate a healthy diesel engine through its compression stroke top-dead-centre is considerable.
Turn the engine over enough times that each cylinder goes through its firing stroke, i.e. one 360 degree rotation per number of cylinders. There should be no obvious difference in effort for any of them. If there is, you can tell which is the faulty cylinder by watching the valves, after removing the valve cover. The firing cylinder is the one that has both valves closed at TDC.
Assuming the rotation effort to be less than expected, the next step is to repeat the test with oil in the bores. Remove the air filter. Turn the engine over on the starter motor with the decompression levers or other stop device activated, whilst at the same time squirting oil into the inlet manifold.
Let the engine sit for a couple of minutes to allow the lubricant to drain down the bores and settle on to the piston rings, where it will make a good, if temporary, seal against the cylinder bores. Rotate the engine again.
If general compression levels are markedly higher there is a strong indication that the rings are worn, whereas a continuing poor compression level suggests that the problem lies with the valves.
Uneven rotation effort both with and without oil added is probably due to one faulty valve but one (or more) badly damaged piston, rings or bore could be the culprit. Taken in conjunction with the evidence of smoke signals you should now have a good idea of the engine's condition.
Vyv Cox is a Chartered Engineer, holding qualifications in both metallurgy and mechanical engineering. Now retired, he worked for 30 years in the petroleum industry, responsible for the diagnosis of engineering failures. Originally based in Anglesey, with wife Jill he cruised extensively between Tobermory and the Scillies. Relocating to Holland they cruised most regions from the Baltic to Biscay in their Sadler 34. They are now meandering eastwards across the Mediterranean and have currently reached Greece.
