How Why To Perform a Compression Test On An Engine

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In this video we're going to show you how to do a compression test. You're probably asking yourself why would you ever need to do a compression test and that's a good question. The reason would be is if your vehicle has a misfire, check engine light that's associated with a misfire, it just doesn't appear to be running correctly and you've ruled other things out. Also, if you're doing a pre-purchase inspection on a car and you want to get an overall idea of the internal health of your engine; that's when you do a compression test.

Some of the tools you'll need are definitely a compression tester. You'll need a socket wrench, and various extensions depending on how it is to get to your spark plugs, the appropriate spark plug socket size, whether it be 5/8ths or 13/16ths. You can see, on our car that we're going to perform the test on here, that it doesn't have an engine cover so we don't need any extra tools to get any fasteners off that way, but again yours might be different. A V8 is mounted this way. That engine cover may need swivels and different extensions to get to those spark plugs, as well as any V8 and 6's that might be mounted this way that have the engine covers on.

There's a couple different ways that this can go. This vehicle has spark plug wires with a distributor. More modern cars have individual ignition coils. In this case, the first thing that we're going to do is remove these spark plug wires and if you have ignition coils you would just simply unscrew them, disconnect them, and pull them out. We're going to pull these out. Just to be safe we're going to disconnect our distributor. If you have a distributor go ahead and disconnect it so that way when you're cranking the car during the compression test you're not getting spark.

The next thing you want to do is locate your fuel pump fuse, or if you can't locate that then the fuel injector fuses and pull them. They could be under the hood or they could be in the cabin compartment. Grab your owner's manual; locate those, pull them. What you don't want to do is be cranking the car and having fuel being sent into the cylinders, washing it out.

In this car our fuse panel with the fuel pump is right here. Go ahead and pull it and then I'm going to go ahead and use my ratchet extension and socket to remove all four of the spark plugs. Go ahead and drop your compression tester in and screw it in until it gets tight, not too tight, though, that you crush the rubber seal on it.

Now going to the tool here, it's in place and it's ready to go. Just a little background here: this is going to test compression in each cylinder one at a time in pounds per square inch. In general we want to be between 140 and 160 pounds per square inch. What you're going to need to do is look up the correct figure for your vehicle, and that way you'll know you're in the right range. I'm going to hold the gauge while I have Paul crank the car. He's going to crank it for about four or five seconds. Okay. As it turns out we've got great compression on the cylinder; about 160. That's good. What we're going to do now is use the decompression and we're going to unscrew it and move on to the next cylinder. We're going to write down the number that we have on the first cylinder so that we can compare it to the rest.

Now we can go over the results. Our first cylinder had 160, the second 150, third 150, and fourth 150. As long as there isn't a deviation of more than 10% between any cylinder it's good to go.

Now go ahead and put your fuel pump fuse back in.

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