Testing compression is a relatively quick and easy way to check the mechanical integrity or health of your engine. A compression test indicates the pressure each of the engine’s cylinders is generating. Excessively low compression in a cylinder is often a symptom of a larger problem, like worn piston rings or valves.
Tools you need for testing compression
- Compression tester (available at most parts stores)
- Ratchet & extensions
- Spark-plug socket
Ensure the engine doesn’t start
A compression test requires cranking – but not starting – the engine. To ensure it doesn’t start, remove the fuel-pump relay, if you can find it. You can also disable the coil by disconnecting the primary power – or the smaller wires – from the coil. Disabling both systems ensures the engine doesn’t receive fuel or spark, meaning it won’t start and the engine won’t flood.
Start with the number-one cylinder. Remove the wire cap and use compressed air to remove any dust or dirt adjacent to the spark plug. Depending on your engine, you may need different lengths and types of extensions to reach the spark plug.
Remove the spark plug
Carefully back out the spark plug. This is a good time for a spark-plug inspection. Examine the electrodes. Ensure the edges are square and the plug looks to be in good condition. It’s a good time to change the plugs if necessary.
Attach the engine compression tester
Thread the compression tester into the cylinder, taking care to avoid cross-threading. Use a compression tester with an extension if possible. An extension allows you to better access difficult-to-reach spark-plug holes. Connect the compression gauge to the extension.
Crank the engine
Have a helper turn over the engine while you watch the needle on the compression gauge. Continue cranking the engine until the needle plateaus, meaning the needle stops climbing. Note the reading on the gauge. Repeat the process for the remaining cylinders. Remember to note compression for each cylinder.
After you’ve tested each cylinder, compare the highest reading with the lowest. A good rule of thumb states they should be within 10 percent of each other. If they are, your engine is healthy and all cylinders are contributing to making power and moving your vehicle down the road.
If psi in one cylinder is significantly lower than the others, for example 20 or 30 psi, it indicates a mechanical problem that requires further diagnosis.