Compression tests for diesel diagnosis

Diesel engines are famously reliable workhorses that rarely complain – but nothing lasts forever. When your old faithful eventually begins to hiccup and burp, a compression test can help diagnose what ails your engine.

Common symptoms of an engine in the early stages of cardiac arrest include excessive smoke at start-up, difficulty in starting, rough running or failing to develop any grunt – ie, it struggles to reach its optimum cruising revs and horsepower rating.

James Mobberley, sales engineer at Auckland’s Moon Engines, says there are multiple possible causes for stuttering performance, and a compression test is a good starting point for a diagnosis.

“The procedure checks the compression in each of the cylinders to verify two key benchmarks: that the compression is within the manufacturer’s specifications and secondly, that the compression is similar across all the cylinders,” he says.

“A compression test is typically carried out on the boat. It is a quick, relatively inexpensive tool for identifying a possible problem or, at the very least, eliminating a number of possible causes in the diagnostic process.”

The test usually involves removing all the injectors and inserting a pressure gauge into each of the apertures – one by one – with the engine cranked for about five to 10 seconds for each cylinder. In engines equipped with glow plugs, the test is even easier. The plugs are all removed and the gauge inserted into each of the apertures, one at a time.

NOTE TO DIYERS

A compression test on a diesel engine is not something you should try yourself, for a variety of reasons. You’ll have to invite a professional mechanic to visit your boat. Performing a compression test on a diesel engine is quite different from performing one on a petrol engine.

Compression tests on petrol engines are easy because the cylinder pressure is relatively low, around 140-175psi. The spark plugs are removed and the compression gauge, equipped with a rubber seat for sealing, is easily held in position during the test. The gauge is inexpensive and available from any auto spares store.

A diesel engine’s cylinders run at much higher pressures, typically between 350 and 550psi. It’s impossible to hold a petrol engine compression gauge in position for a diesel engine test. Remember: a diesel fuel mixture ignites from the heat generated by the high compression; a petrol engine’s fuel mixture is ignited by spark plugs.

Furthermore, a diesel engine compression test usually requires the removal of all the injectors. This means the fuel injection system has to be reassembled and bled after the test – quite a complicated, lengthy process. The compression gauges are pricey,