Compression testing of petrol engines
Apologies for the length of the posting, its split into three posts so you can either jump straight into the method in brief or you can follow the method in step-by-step detail. The third post deals with examining the results. Enjoy the read, happy testing and may the force (pressure) be with you.
When to carry out a compression test
If the engine is running rough or is suffering from a lack of power or performance and no faults can be found in the ignition circuit, fuel system, crankcase ventilation system, or the exhaust system. Carry out a compression test to confirm or eliminate worn piston rings/cylinder bore, worn valve gear/camshaft or blown head gasket/cracked head.
If you’ve just taken delivery of a ‘new’ car, OK in reality it may be 10+ years old with 100K+ miles on the clock. Carrying out a compression test will give you a good idea of the overall condition of the engines internal components, the bits you don’t notice when walking round and kicking wheels.
If you’ve just ragged it quarter after quarter at a Santa Pod RWYB day, or maybe you’ve just put in a personal best at a Snetterton track day, you might just want to check you’ve not done any internal damage and it’ll still get you to work and back on Monday.
If you’ve added some performance parts such as camshafts, a skimmed or modified cylinder head, modified pistons and the like. Carry out a compression test to give yourself a reference point of what sort of pressure the combustion chamber is producing. All these things will have some effect, modified camshafts may lower the pressure, skimmed or modified heads may raise it.
Incorporate a compression test into your normal service routine when you change the spark plugs, half the works done once the plugs are out and it’s not so messy changing that fuel filter when its empty! This will give you an early indication of any internal wear and you may just pick up on an early sign of a blown head gasket before it becomes a cracked cylinder head. If you do this keep records of your tests showing date and mileage for future comparison.
Calculator, pencil & paper, yellow green & red PVC tape (optional), 5mm hex key (if your plugs have a cover), spark plug socket, extension and tee bar or ratchet, 50ml of engine oil if you need to wet test, and, of course, a compression tester. I recommend having a workshop manual to hand for reference. You will also need a helping hand when it comes to spinning the engine over.
If you haven’t got a compression tester a trip to your local motor factor with £15-£20 will get you sorted. The screw in type are the easiest to use and offer greater accuracy over the rubber bung type. They comprise of a pressure gauge calibrated to about 20 Bar (or 300 psi), non-return valve (which leaks a little), rubber hosepipe, one-way valve (also leaks a little) and varying sized spark plug hole adaptors.
The method in brief
Calculate the pressure figures, note to carry out a proper test you will need to take three or four pressure readings for each cylinder. Working on a warm engine with a fully charged battery, disable the fuel system, isolate the ignition circuit and remove the spark plugs. Screw the compression tester into the No1 cylinder spark plug hole, spin the engine on full throttle, note and record the pressures. Repeat the test on the remaining cylinders, noting and recording the pressures. Examine the results, wet test if required. Re-fit the spark plugs; connect the HT & LT leads, re-fit fuel pump fuse. See below for step-by-step detail
Time allowed 20 minutes, allow another 10 minutes if you carry out a ‘wet’ test too.