How to Do a Compression Check on Your Engine

Posted: September 24, 2020 By: MattM

Low or inconsistent engine compression can be the cause of some major issues in your vehicles engine. A quick check of compression can give you an idea if you have a major issue or something else more simple is wrong. Below we cover the basic steps for doing a compression check yourself at home.

  1. Remove/Disable Fuel and Ignition System- When doing compression test you DO NOT want your engine to start and run. We just want to crank the engine over enough to build compression. Depending on the age of the car this may just be a matter of disconnection the coil wire off coil or disconnecting the fuse for your ignition system. Obviously with a carburetor you can’t disconnect the carb, but on a fuel injected car you can also disconnect the fuse for the fuel pump. At minimum you should disconnect the ignition system; but it doesn’t hurt to disable both.
  2. Remove Spark Plugs- If you’re using the starter to spin the engine over you can simply remove the spark plug from the cylinder you want to check the compression on. It’s a good practice to use compressed air to blow any debris away from the spark plug opening BEFORE you remove the plug.
  3. Attach Compression Gauge- Thread your compression gauge fitting into the spark plug hole. Most compression gauges have a rubber o-ring that allows you to tighten the fitting down by hand and easily seal the gauge up.
  4. Turn Engine Over- With the car in park or neutral with the handbrake on you can crank the engine over. Turn the key and spin the engine over like you’re starting the car. Listen to the engine crank over and let the engine spin over 3-5 times before letting off of the key. You can then check the compression gauge. If you have a helper they can also watch the gauge and let you know when the gauge stops rising. Note what number cylinder that was and the compression reading and move to the next cylinder.

After checking each cylinder review your readings and note if any cylinders have a large variance in the reading. This cylinder may be a problem. If any of the cylinders read under 100PSI you may have a worn engine or a problem on those cylinders. If you pour a small amount of oil down the questioable cylinder and retest and the reading is significantly high you may have a worn cylinder or rings. If that doesn’t change anything you could have issues on the valve train side. It is a good idea to check compression on your classic car periodically to catch any engine issues before they become a major problem. If you need to invest in a compression gauge you can find the Eastwood Compression Gauge Kit HERE.

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