Interpreting Your Auto Motor Compression Tests

Auto cylinder compression tests and testing can be the simplest, easiest and overall quick evaluations of the condition of the motor in a vehicle that you can do. On top of that compression testing of an auto engine is fast, cheap and what might be called “noninvasive” that is you do not have to take apart half of the engine.

Yet many inexperienced and backyard mechanics often do not know the first thing about interpreting results of compression tests. They may know how to complete the full test – yet really are in the dark about what results mean.

First – take the time and effort to compare the compression readings with the car makers’ specifications and specs. These can be obtained either from the manual, a source book, online at the auto manufacturers’ website or as a last resort your neighborhood garage mechanic or dealership service department. Basically your readings should be within the tolerances allowed.

In the situation and event that your engine does show low levels and readings of compression. That is less than 100 psi (pounds per square inch) or alternatively high compression readings, then you can almost count on the eventuality internal engine repair work is probably necessary and most likely is around the corner. If the compression testing was done as standard workup in routine maintenance and you were considering an automotive motor tune-up, then it may well be a good use of time and your resources to lay that idea aside. Go work on another vehicle.

If the compression readings are low or uneven in any of the motors’ cylinders, a mechanics tip is to add a tablespoon of engine oil through the spark plug openings of the affected cylinders and then recheck the compression. If the compression readings improve, then it is most likely the case that the cylinder valves are probably “sticking”, seated poorly or alternatively burned. The experienced owner of a string of auto dealerships whose practical expertise in the auto and auto body shop industry states that in his experience valves are bad ten times more often than rings. If the compression is low in two adjacent cylinders, the cylinder head gaskets between these cylinders are mot likely blown. Alternatively high compression readings usually indicate a buildup of carbon deposits in the compression chambers of the cylinder of the engine.

Lastly when you have finished testing the compression, turn the ignition switch off and remove any instruments with which you have lodged or held the gas pedal in the open position. You now not only have completed the full compression testing but also are a much better situation to understand and comprehend the meaning and significance of results obtained.