Compression Testing Guide
What you should know before you buy a used car

Tools Needed:

Spark Plug Socket	Socket Extension
Wrenches	Torque Wrench
Pen	Paper

This is a quick start guide on compression testing your engine.

Aside from a vehicle history report, a compression test can be one of the most useful pieces of information you can have when considering buying a new car. Compression tests take less than thirty minutes and can identify engine problems such as worn rings or valve seats.

The pictures in this guide were taken on a Volvo 850. Though component locations may vary, the information here can be applied to any petrol car. With the car fully warmed up, I usually drive it for at least 10 minutes, start by removing the fuel pump fuse and ignition coil wire. If it is easier you can also remove the harnesses from the fuel injectors instead of the fuel pump fuse.

Below you can see the fuel pump fuse and coil wire disconnected. With this done, remove the spark plug cover. On Saab and Volvos this cover is secured with several T30 torx.

With clear access to the plugs gently pull straight back on the plug wires. If you have coil packs you will need to remove a pair of bolts before you can do this. With a spark plug socket remove the plugs noting which cylinder they came from.

While the plugs are out, inspect them for any abnormal wear or deposits. The plugs below are in generally good shape but do shows signs of lead and MMT exposure.

You are now ready to start the compression test. Have pen and paper ready to record your results. To start, screw in the testers hose to the first cylinder. Now crank the car 5-6 cranks. Record the results and repeat once more to verify the results. If both numbers are close, average them and move on to the next cylinder until you are complete. A healthy engine will have all the cylinders within ~5% of each other.

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Here is a short movie that highlights the process.

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