Running Compression Test

Normal Running Compression

Most people have heard of a compression test. A compression test can give a service technician a good picture of the internal health of an engine. In this post I will tell you about a little used type of compression test that can give a little more insight to the internal interaction between your pistons, valves and camshafts.

    We need to have compression inside the engine to squeeze the gas and air mixture to such a state that only a spark (from the spark plug) will start the burning process. This burning process is what pushes the piston down into the cylinder to power your vehicle. A misfire is what occurs when the burning process does not take place.

A regular compression test is performed without the engine running (just cranking the engine over). When regular compression, fuel & air mixture and spark are OK but the engine is still misfiring I want to perform a running compression test. As the name implies, a running compression test is performed with the engine running.

 

Cylinder With Compression Loss

In this example the running compression peaks out at around 75 lbs. The engine being tested is misfiring on this cylinder. The actual compression reading is not compared to other engines but to another cylinder on the same engine. On the good cylinder running compression is around 100 lbs. Our conclusion is that this engine will require tear down and inspection to address an internal pumping problem.