No Need To Break The Bank When Picking A Compression Gauge

Dear Car Talk:

Can you recommend a good compression test gauge for a shade tree mechanic? -- Kevin

Interestingly, I can't even remember the last time I used a compression tester, Kevin. So, if you can find mine, you can have it.

Cars just don't burn through valves and piston rings like they used to. So, we rarely see cars with compression issues these days. Plus, we do a lot of diagnosis using our scan tool. The scan tool can tell you, for instance, if there's gasoline that's not being combusted in one or more of the cylinders. But to find out why, you have to dig in and start taking things apart.

If you suspect that the problem is bad compression -- due to a bad valve, a worn piston ring or a badly blown head gasket -- a compression test will give you a definitive answer.

An engine works by compressing the air and fuel mixture inside the cylinder. If the compression doesn't get high enough, the mixture won't combust well, and the engine will run poorly -- or not at all.

So, to test each cylinder's compression, you remove the spark plugs and thread the compression gauge into one of the spark plug holes. Then you crank the engine, as if you were starting it, and check the reading. And you do that for each cylinder.

A compression gauge doesn't have to be a precision instrument, because what you're really looking for is a big difference between the cylinders. So if three cylinders are at 150 pounds and one is at 80 pounds, you know there's a problem. If the absolute numbers are off a bit, it hardly matters.

If you go online, or to your local auto parts store, Kevin, you should see a wide selection of compression gauges for sale. You really don't need to spend more than $30 or $40 to get one that'll do the job.

Look for one with a hose, which makes it a little more versatile and easier to use. And then, once you've checked your own compression, your wife's, and your father-in-law's, maybe you can put up a yard sign that says "free compression tests" and make some new friends.

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