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How to Perform a Compression Test on the Engine in a Ford F-Series

by Contributor

The Ford F-150 contains either a V6 or a V8 engine, while the F-250 and F-350 trucks feature the choice of a V8 or V10 engine. For the 2008 model year, Ford also introduced the F-450 truck, which contains a V8 engine. When the engine in one of these powerful trucks is running poorly, losing power or burning oil, a compression test is an easy way to diagnose the problem.

Warm up the engine in the Ford truck completely before performing the compression test. Running the test on a cold engine produces inaccurate results. Be sure that the engine in the truck is in good shape since it will need to spin the engine several times during the test.

Disable the ignition system to avoid the possibility of electrical shock during the compression test and remove all of the spark plugs from the engine. If the truck contains a V6 engine, there will be only six spark plugs, but if it contains a larger V8 or V10 engine, there will be either eight or ten spark plugs. Inspect the spark plugs for damage since this is a sign of problems within that cylinder.

Insert the compression tester into one of the spark plug holes and crank the starter. This rotates the engine and builds compression within the cylinder. Record the compression number and repeat the procedure for each of the remaining cylinders. There will be a record of six, eight or ten numbers when you finish, depending on the engine size.

Compare the compression numbers to the recommended compression ratio for a Ford F-series truck to see if they fall within 15 PSI of that number. For an F-150 with a V6 engine, the compression ratio is 9.2:1, and for one with a 5.4L V8 engine, the ratio is 9.8:1. For both F-250 and F-350 trucks with a 6.0L V8 engine, the ratio is 18.0:1, but if the truck contains a V10 engine, the ratio for 2007 and 2008 models of these trucks is 9.2:1 and 9.0:1 for the 2004 through 2006 models. Compression ratios vary for all other V8 engines in these models by year and liter.

Evaluate the results by comparing the compression numbers to each other as well. They should be similar and fall within 10 PSI of each other. Retest any cylinders with a low reading after adding a small amount of motor oil to the cylinder. If the number increases, the cylinder contains worn walls or piston rings, but if it stays the same, there are worn valves or valve seats instead.

Tips

  • For 2006 through 2008 models of the F-150 with a 4.6L V8 engine, the compression ratio is 9.4:1 but changes to 9.3:1 for the 2004 and 2005 model years.
  • For the Ford F-450 truck, the compression ratio is 16.7:1.
  • Compression ratios for the Ford F-series truck models before 2004 vary, so check the owner's manual for the correct ratio.

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