Does your car engine need a compression test?

The hows and whys of this tool
by Niky Tamayo | Jun 20, 2017

The internal combustion engine (ICE) is a marvelous thing, harnessing the power of liquid hydrocarbons–ergo: gasoline or diesel fuel–to create motive force for motor vehicles. But over the years, after hundreds of thousands of kilometers and millions of engine revolutions, your car’s engine can start to feel a little tired.

If you’re experiencing a loss of power that can’t be fixed with new spark plugs, injectors, or electronic sensors, there is a chance that there’s something more fundamentally wrong with your engine. An ICE is basically a big air pump and compressor, sucking in huge amounts of air, mixing it with tiny amounts of fuel, and squeezing it into a very small combustion chamber using a piston. When ignited, this mixture pushes the piston back down, turning the engine crank and pushing the car forward.

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Like any pump, achieving the proper pressure requires a good seal around the piston. Air and water pumps, like your common squirt gun, syringe, or bicycle pump, use rubber gaskets. A car’s engine requires tougher stuff. Each piston inside a piston engine is sealed against the cylinder wall with metal piston rings, while metallic gaskets, valves, and valve seats are required to ensure proper pressure in the extremely hot combustion chamber.

While quite tough, these components do wear down over time. If your car is losing power and/or emitting more smoke than usual, it may be time for a compression test.

A compression tester is a disarmingly simple tool. It’s a pressure gauge with attachments that allow you to plug it into a spark plug or diesel injector socket. Once there, both ignition and fuel systems are disconnected and the engine is cranked (via the starter motor) a few times. This procedure is repeated for each cylinder, after which the readings are compared to each other. This is preferably done on a warm engine, as piston rings often don’t seal properly when cold.

Engines naturally lose compression over time due to wear and tear, but having compression that’s a bit lower than it was when brand new–typically 180-220+ pounds per square inch of pressure–is okay. What you’re looking for during the test is abnormally low numbers or numbers which don’t line up.

If one or more pistons are making 10% less compression than the others, then it’s time to do a wet test. A little bit of oil is added to the offending cylinders, then the compression is checked again. If the compression improves, then the piston rings are not sealing properly. If it doesn’t, then there’s a chance that your headgasket or valves are at fault.

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Another follow-up test is the leakdown test. With the offending piston at top dead-center–or the top of its stroke inside the engine–and the valves closed, air is pumped into the cylinder. The mechanic then checks or listens for leaks. Air hissing out of the dipstick tube or crankcase vent on top of the engine indicate piston ring issues. Hissing from the exhaust or intake indicates valve issues. Bubbles coming out of the coolant, on the other hand, indicate a broken headgasket or possibly a cracked or warped cylinder head.

None of these issues are cheap to fix, and all will mean some down time for your car. But with a proper diagnosis, you can catch these problems before they become even more expensive. And as long as you familiarize yourself with compression testing and the expected results for your particular vehicle, you can at least assure yourself you’re not paying for unneeded repairs, and that your mechanic really knows what he’s doing.

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PHOTO: Jason Tulio
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