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Engine Compression Test
By Mike Bumbeck/autoMedia.com
Printable version
Degree of DifficultyModerate
Moderate
Estimated Time180 minutes
180 minutes
 
Squeezing the Most Out of Your Engine -
 
An automobile engine's most crucial task is to make power. The engine performs this task by taking in a mixture of fuel and air, pushing the mixture into a confined space, setting the mixture aflame, and harnessing the energy from the combustion and redirecting it into the power that gets you moving down the road. The ability of that confined space to contain the energy of the combustion is what enables an engine to utilize the energy instead of letting it escape uselessly out of the confinement. The valves and seals, pistons and rings, and cylinder walls all work together to create a tight seal so that the burst of energy pushes the piston back down into the cylinder.
Engine Compression Test
This power stroke is the key to how a four-stroke engine turns combustion energy into useable power. If any or all of the parts that confine the combustion inside the cylinder become worn, engine performance suffers twofold. First, the engine cannot compress the fuel and air mixture enough to get the required big bang. Second the bursting mixture escapes past worn valves and seals, pistons and rings, and worn cylinder walls. Not only does the engine suffer a power loss, but also burns the fuel less efficiently. The engine may also begin to burn oil. The same worn rings and seals that let the burst escape, also allow oil to get in. An engine compression test checks cylinder sealing ability one by one, and can help isolate internal engine problems.
 
Fourth of July
The parts that comprise the combustion chamber act similar to the tightly wound paper around the gunpowder inside a firecracker. The gunpowder inside the firecracker is like the compressed air and fuel mixture. Light the firecracker and it goes bang. The reason it goes bang is that the energy of the burning gunpowder, for an instant before the bang, has nowhere to go. The layers of paper wrapped around it are containing it. Eventually the energy contained within the burning gunpowder overcomes the tightly wound paper around it and blows a hole in the side of the paper. Bang! If one were to take the same small amount of gunpowder that usually goes inside a firecracker, place it a little pile all by itself and light it on fire, the result would be much less dramatic. It would merely fizzle a little, and stink quite a bit, for there is nothing to confine the energy of the burning powder.
 
The automobile engine holds in the energy of the bursting air fuel mixture much like the layers of paper around the firecracker. Since a bursting air fuel mixture blowing a hole in the side of an engine would not be a welcome thing, automobile engines are built to harness the combustion of the energy into a downward motion. The piston moves down into the cylinder as the mixture combusts. Since the pistons are all connected to a spinning crankshaft, the energy of the combustion is harnessed into power by the transmission—and away you go to watch the fireworks with a picnic basket in the trunk.
 
Testing Day
There are some general guidelines to follow when running a compression test on a four-stroke gasoline engine. Rotary, two stroke, diesel, and other types of engines require a different procedure. It's always a good bet to follow testing steps recommended in a service manual before drawing any conclusions based on test results. The service manual will also have compression service limit numbers.
 
Make certain the battery is in good shape, as it will have to spin the engine quite a few times. Warm up the engine. The reason for this is that heat makes all the metal parts inside the cylinder expand and seal up better. Having things warmed up will give a more accurate compression reading. A cold engine will give inaccurate compression readings. Stop the engine, and remove all the spark plugs. This way the starter can spin the engine freely. Disable the ignition system by either pulling the coil wire, or disabling the ignition coil. To perform the actual test, insert the compression tester into one of the spark plug holes and crank the starter to rotate the engine in order to build compression in that cylinder. There may also be other required steps, such as disabling the fuel system or holding the throttle open. Record the compression number for that cylinder. Move onto the next cylinder until numbers have been recorded for each cylinder. Once all the numbers have been recorded they can be compared, and an overall conclusion can be drawn.
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