Measure each cylinder with the engine at normal cranking speed and the correct adapter fitted, and compare cylinders against each other as well as against specification. A single low cylinder indicates a valve or piston problem on that cylinder; all cylinders low indicates general wear.
How to confirm it
- Eliminate fuel and air faults first
Compression testing requires dismantling the fuel system access and is far more work than checking filtration, bleeding the fuel system or testing the shutdown solenoid. Establish that the engine is receiving fuel and air before going further.
- Warm the engine if it will run
Compression rises with temperature and stabilizes with warm components. A cold engine gives lower and less consistent figures. If the engine will not run at all, note that the figures are cold readings and compare cylinders against each other rather than against absolute specification.
- Isolate the fuel system correctly for the test
Disable injection according to the manufacturer's procedure. On common rail systems this means more than pulling a fuse; follow the specified method to avoid injecting fuel during the test or storing fault codes and damaging components.
- Fit the correct adapter and a diesel-rated compression gauge
Adapters and gauge ranges are engine-specific. Using a gauge rated for gasoline engines, or an adapter that does not seal properly at the injector or glow plug port, produces low and misleading readings on every cylinder.
- Crank at normal cranking speed and record each cylinder
Crank until the gauge reading stabilizes, and record the reading for each cylinder by number. Cranking speed matters: a weak battery or starter gives uniformly low readings that look like a worn engine but are not.
- Compare cylinders and interpret
Compare each cylinder against the others and against specification. One or two low cylinders indicate a specific fault: a burnt valve, a broken ring, a scored bore or a gasket failure between adjacent cylinders. All cylinders uniformly low indicates general wear or a test fault such as low cranking speed.
- Measure blow-by as a cross-check where possible
Blow-by measured at the crankcase breather indicates how much combustion gas is passing the rings, and it can be checked without dismantling anything. It is a useful confirmation and sometimes a better guide to overall wear than a compression figure alone.
What to do about it
One cylinder low with adjacent cylinders normal: a valve problem or piston problem local to that cylinder. A cylinder leak-down test on that cylinder distinguishes valve sealing from ring sealing before the head comes off.
Two adjacent cylinders low together: a head gasket failure between them. This is consistent with coolant loss and white smoke, and the head should come off for inspection rather than the engine being run further.
All cylinders low: general wear, or a test artefact. Confirm cranking speed and gauge calibration before accepting a worn engine diagnosis, because a weak battery has caused many unnecessary engine overhauls.
Low compression with blow-by: piston rings and liners. A liner kit is the standard repair, and the correct kit is identified from the engine model and serial number rather than from the machine model alone.
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Request a quoteFrequently asked
Can I do a compression test on a common rail engine?
Yes, but the fuel system must be isolated by the manufacturer's specified method and this is not the same as pulling a fuse. High-pressure fuel systems require care, and the procedure differs between platforms.
Is blow-by as good as a compression test?
It answers a related question with far less work. Blow-by tells you how much gas is escaping past the rings; compression tells you what pressure the cylinder can build. For assessing overall wear, blow-by is often the more practical check, and it can be repeated easily to track a trend.
Why are all my readings low?
Most commonly low cranking speed from a weak battery, starter or cables, or a gauge or adapter that does not seal. Check those before accepting a worn engine diagnosis — this mistake has cost a great many owners an engine overhaul they did not need.