A running (dynamic) compression test confirms; how efficiently each cylinder is pulling air in; retaining it for the correct amount of time; then releasing it into the exhaust.
So, if a cylinder cannot perform these functions properly; the result can be a loss of volumetric efficiency or a misfire.
In my opinion, you should NEVER do any test procedure unless you understand why you are doing that test. As well as what you will do with the results.
A running (dynamic) compression test; can also be used to pinpoint the cause of a misfire. Even more, when all the usual tests return normal results.
So, this test should not be confused with similar tests; that are used for different results:
- When you do a cranking (static) compression test, you are checking cylinder sealing, not cylinder breathing.
- When you test engine vacuum at the manifold, you are looking at the breathing of the entire engine; by checking vacuum at a common (plenum) source.
- Also, when you do a cylinder leak down test, you are actually checking cylinder leakage.
- A cylinder power balance test measures the power that a particular cylinder; contributes to the total power of the engine.
A running (dynamic) compression test uses a compression gauge, to check individual cylinder breathing. So, basically you are checking how well your engine is inhaling and exhaling.
NOTE: You could use a Lab Scope and Pressure Transducer so you can see the results. But, we are going to perform this test, using a basic engine compression testing gauge.
This test does combine some other testing as well. You will be looking at the overall results, as well as comparing measurements between cylinders.
How To Do A Running (dynamic) Compression Test
The first thing you need to do is create a chart, similar to this example to record your test readings.
Use this chart to record all the compression information from the CRANKING-(STATIC), IDLE and SNAP tests. As a result, we will be comparing measurements between cylinders as well, to determine what it all means:
- Start with a cranking (static) compression test on a warm engine to eliminate obvious problems; such as badly worn rings, damaged cylinders, burnt or bent valves or holes in pistons. If you have a diagnostic trouble code, you may know which cylinder is acting up. But, it’s still a good idea to test the compression of all the cylinders, to get a good comparison. Record your cranking (static) compression readings on the chart you made earlier.
- Now, reinstall all but one spark plug.
WARNING: GROUND THAT PLUG WIRE TO PREVENT MODULE DAMAGE; OR WHEN EQUIPPED WITH A COIL-ON-PLUG IGNITION; SIMPLY DISCONNECT THE COIL HARNESS PLUG. ALSO, DISCONNECT THAT INJECTOR ON A PORT FUEL SYSTEM:
- Install your compression tester into the empty hole. The test can be done without a Schrader valve but; most people recommended leaving the valve in the gauge. Then, “burping” the gauge every 5-6 “puffs”.
- Start the engine and take an “Idle” reading. Be sure the idle speed is a consistent (rpm). Record the results on the chart.
- Now, from the idle speed (rpm), snap the throttle to 2500 (rpm) and release quickly. The reading should rise. Record the results.
NOTE: Don’t use the gas pedal for this snap acceleration. The idea is to manually open then close the throttle as fast as possible. As a result, forcing the engine to take a “gulp” of air. Note: This could be a challenge with (Electronic Throttle Control) systems.
Basic Testing Information
- Cranking (Static) Compression Value: Base Formula: Compression Ratio x 14.7 = (manufacturers spec). Example: Manufacturers Spec of 9:1 x 14.7 = 133.7 psi (Compare with manufacturer specification to be sure)
- Running (dynamic) Compression At Idle Speed: Should be 50-75 (PSI) (about half of cranking compression).
- Snap Throttle Compression: Should be (about 80% of cranking compression).
To analyze the results we have to compare measurements, between all cylinders.
Interpreting The Test Results
Cranking (Static) Test Results:
- Compare to manufacturer’s specs. Allow no more than 10% variation between cylinder.
Running (Dynamic) Test Results:
- Running (dynamic) compression at idle should be 50-75 psi (about half of cranking (static) compression).
Snap Compression Test Results
- Finally, test results from the snap test should be about 80% of the cranking (static) compression test.
A Low Snap Test Reading
If a snap reading is low (much less than 80% of cranking compression); look for air intake problems.
- Severe carbon deposits on intake valves.
- Worn camshaft lobes.
- Worn valve guides and springs.
- Rocker or push rod problems.
- “shutter valve” miss-positioned in the runners of a, variable runner intake system.
A High Snap Test Reading
If a Snap measurement is significantly higher (over 80% of cranking compression); it means the air is not leaving the cylinder efficiently. Look for problems on the exhaust side.
- Worn cam lobes.
- Bent pushrods.
- Collapsed lifters.
But, if the snap readings are all high, look for exhaust restrictions. Such as a clogged catalytic converter or muffler.
A Low Idle And Snap Test Reading
So, these types of numbers indicate that the cylinder is (not holding compression efficiently). Look for issues.
- Slightly bent or burned valves.
- Excessive carbon buildup on valves or seats.
- Worn valve guides and springs.
- Scored cylinder walls.
- A leaking head gasket.
NOTE: Consequently, a cylinder leak down test will narrow down, where the leakage is coming from.
So, as you can see, information gathered from a running (dynamic) compression test; can be very helpful when diagnosing misfires issues. Just, another great test to help solve engine problems.
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