Compression Testing an Engine

1.  Please read this entire article about analyzing engines.

2.  When you have completed the article, I want you and your team to choose one of the laboratory vehicles or a live job with the instructors approval, and complete the engine evaluation.

3.  Complete the Repair order and furnish it and the technician notes to the instructor for evaluation. For help follow these guidelines click url for help.
Repair Order Writing NU
4. Finally, post your own findings, including your final analysis, problems you encountered and what you learned about the engine analysis process. Post your comment by clicking on the comments listing at the end of this article.

                                                                                       

Compression and Leakage Testing Notes            

When compression or leakage testing an engine the engine should be warm with a fully charged battery, and the throttle plate blocked open for full air flow. 

          The result specifications will vary by what you are testing for and if you are working for a manufacturer considering warranty returns.  You must follow these specifications to be in compliance with your employer. Usually between 15% for High Performance and as low as 30% for Low Performance/Utility.  In my Independent shop, I personally followed these specifications while working for my customers and they were quite content with my outcomes and charges for that work.  A well running engine had all cylinders within 20% of one another, and 20% maximum leakage. 

Compression Testing

A difference of more than the 15% Performance to 30% Utility would be a major concern.  All spark plugs removed.  The first pressure impulse would be normally at least 50% of the final reading.  Since 1997 nationally, 4 impulses should be the normal amount of cranking during testing (Aspire, DCA/BAR 1996).

Wet Testing a Failing Cylinder

For gasoline engines that have failing cylinders a wet test and identify top end problems (Valves) or bottom end problems (Rings).  This test is a quick test that does a fair job of identification.  A preferable test is the Leakage test.  Use a small amount of oil injected into the cylinder, 2 squirts of an oil squirt can through the spark plug hole. (Not on diesel engines)  A substantial increase in pressure will indicate failing rings, by artificially sealing the rings temporarily to the cylinder wall.

Cylinder Leakage Testing

The Cylinder Leakage Tester in conjunction with a Compression test is an excellent way to diagnose specific failures in a cylinder.  I personally choose to remove only one plug at a time, Snap-On likes to remove all plugs at once to hear leaks through adjacent cylinders, head gasket was however checked during the compression test. Absolutely new engines leak at 7%, broken in new engines leak at 4%, maximum leakage below 15% Performance, and 30% Utility, depending on customer is allowable.                

                                                                                      “MAC” 7/11