A used diesel is often removed because it smokes, knocks, or will not start. Those symptoms have cheap causes and expensive ones. A compression test separates them. It is the one measurement we would rather see from your workshop than a long description of the noise.
When to do it
Do it while the old engine is still in the car and will crank. Once it is on the floor, the test is harder and people skip it. The test needs a warm engine, a charged battery, the injectors or glow plugs removed, and the fuel system disabled so it does not fill the cylinders with diesel while you crank.
How to read it
Compare the cylinders with each other. A healthy six-cylinder diesel shows six readings close together. One cylinder far below the others means a ring, a valve or a head gasket at that bore, and another engine is usually the right answer. All six equally low can be a timing or a test error. Repeat the test before you condemn the block.
We do not publish a single "pass" bar in bar or psi. Gauges differ, and a cold test lies. Even readings, taken properly, are the result that matters. A leak-down test is the next step if one cylinder is down and you need to know whether the air is escaping past the rings or the valves.
What we cannot do on a pallet
Engines in this yard are drained and standing on pallets. We do not crank them. A compression figure on our listing would be invented, so there is not one. What we can show is the code, the sump, the turbo and the oil that came out. If you need a tested running engine, say that before you order. We would rather tell you we cannot start it here than imply a test we did not do.
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A worked example
Six cylinders at roughly the same figure, and one cylinder a long way down, is a mechanical fault in that bore. Six cylinders a little apart, on a cold engine, is a test to repeat when hot. A test done with one injector still fitted, or with a flat battery, is not a test.
Write the six numbers on the job card and keep them with the invoice for the replacement. If the new engine is ever questioned, those numbers show why the old one came out.
The yard is at Andreas Meyer Straße 38, 22113 Hamburg. Call or WhatsApp +49 15510 030835, or email autowerkeengines@gmail.com, Monday to Friday, 9:00 to 16:00 CET. Shipping is 249 € per order. We are an independent seller, not BMW AG.