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How to Use a Compression Tester

Test all the cylinders in your internal combustion engine with a compression tester to narrow down the cause of problems.

The internal combustion engine has four strokes: intake, compression, combustion and exhaust. Each step requires the precise timing of several components working together to create the perfect environment to keep the engine running. And each stage is integral to the process — if something is off, your car will have a way of letting you know. Compression problems aren’t super-common, but when they occur, you’re already looking at significant repairs. If you use a compression tester to diagnose them early on, it can save you a lot of time and hassle down the road.

When Something’s Wrong

Low compression can be caused by a multitude of problems, from worn piston rings to stuck valves to a blown head gasket to a broken timing belt. And more! You might notice a misfire, lower power climbing hills, or perhaps your car won’t start at all. Compression problems definitely shouldn’t be your first suspect in most of these scenarios, but if you’ve ruled out other causes, or if, when you go to start the engine, it sounds like it cranks really, really fast, get yourself a compression tester.

Disassembly Required

How to Use a Compression Tester

Start with a cold engine. Disconnect the fuel pump relay and the main wire to the ignition coil. This will prevent fuel and spark from mucking up the process. Next, make note of the order and location of spark plug wires before removing them — they must be placed back in the same order later. Then, remove the spark plugs, taking care not to damage the ends. Keep them in order, as well, and inspect them individually for clues that something’s up in the combustion chamber.

Testing for Less

Starting with cylinder 1 (you may have to refer to your owner’s manual to find the cylinder ordering numbers), thread the compression adapter into the spark plug hole. Most compression testers come in two or more pieces that should be threaded or snapped together from the parts closest to the engine out — all only by hand. Next, have a friend crank the engine for five seconds or so while you observe the gauge. You’re looking for the highest number reached, typically anywhere from 125 to 160 psi on most vehicles. Make a note of this number, disconnect the tester and repeat for all cylinders. If one cylinder falls significantly below the others, you’ve narrowed the compression issue to something related to that cylinder. If all measurements are low, then your problem lies with something like the timing chain or a major mechanical failure.

In any case, head on over to your local NAPA AutoCare for a more comprehensive diagnosis, because low compression is no joke, and continuing to drive (if you even can) is going to do a lot of harm. This is one problem to get taken care of immediately before bad turns to worse.

Check out all the tools & equipment available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on compression testers, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Flickr.


Blair Lampe View All

Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter.  In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.

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