How to Perform a Compression Test on the Engine in a GMC Sierra
Although a V6 engine is available, most GMC Sierra trucks come with a powerful V8 engine, and it is important to keep them running properly to get the most out of these engines. A compression test is a simple way to find out what happens inside the engine without tearing it apart. Worn piston rings, cylinder walls and valves are common problems diagnosed by performing a compression test.
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Instructions
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Warm up the engine in the GMC Sierra before beginning the test, since a compression test performed on a cold engine will be inaccurate. The battery should also be in good condition, since it will spin the engine several times.
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Remove all eight spark plugs from the V8 engine after you turn it off. Models containing a V6 engine have only six spark plugs. Disable the ignition system to eliminate the possibility of electrical shock while performing the compression test.
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3
Place the compression tester into one of the spark plug holes and crank the starter so that compression builds in that cylinder. Write down the compression number and continue to the next spark plug hole. Repeat the procedure for each of the six or eight cylinders, recording each number.
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4
Compare the compression numbers to manufacturer specifications. If the GMC Sierra contains a 4.3L V6 engine, the ideal compression ratio is 9.2:1. The ratio varies by engine size in trucks with a V8 engine. If it contains a 4.8L or 5.3L engine, 9.5:1 is ideal. If it has a 6.0L engine, the ratio should be 9.4:1.
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Evaluate the difference between the recommended compression ratio and the test results. An ideal reading falls within 15 PSI. The numbers should be within 10 PSI of each other as well. You will need to retest any cylinder with a low reading.
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Add a tiny amount of motor oil to the cylinder with the low reading before running the compression test again. If the resulting number is higher than the first, the cylinder has worn cylinder walls or piston rings. If the number remains the same, it has worn valves or valve seats.
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Tips & Warnings
The compression ratios listed above refer to GMC Sierra trucks made between 1999 and 2006. When performing a compression test on a newer model, the recommended numbers change for all engines except the 4.3L V6 engine.
