How to Perform a Compression Test on the Engine in a Ford Focus
A compression test is a helpful tool used to diagnose engine trouble. It's easy to perform and doesn't require tearing apart the engine to see what isn't working properly inside. Some common problems revealed by performing this simple test on the engine in a Ford Focus include worn valves, piston rings and cylinder walls.
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Instructions
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Warm up the engine in the Ford Focus, since a compression test run on a cold engine will not be accurate. Be sure that the car battery is in good condition, since it will need to spin the four-cylinder engine several times.
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Turn off the car and remove all four spark plugs from the engine; now the starter can spin the engine freely. Disable the ignition system to avoid electrical shock.
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Place the compression tester into one of the spark plug holes and crank the starter to rotate the engine. This builds compression in that cylinder. Record the resulting compression number for comparison later. Repeat this process for each of the four cylinders, keeping a record of all the compression numbers.
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Check to see if these numbers fall within 15 PSI of manufacturer specifications. If the Ford Focus contains a 2.3L engine, an ideal reading is 9.7:1. If it has a 2.0L engine and is a 2005 model or newer, the compression ratio is 10.0:1 instead. In older models with a 2.0L engine, the recommended compression ratio varies by year and engine, so be sure to check the vehicle manual.
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Compare your numbers to each other as well. Each number should fall within 10 PSI of the others. If one cylinder shows a low reading, add a small amount of motor oil to that cylinder and perform the compression test again. If the compression number increases, worn cylinder walls or piston rings are the problem. If the number doesn't change, the engine has worn valves or valve seats instead.
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