Running ( Dynamic ) Compression Test – Evaluating individual cylinders.

Running ( Dynamic ) Compression Test - Evaluating individual cylinders.
Running ( Dynamic ) Compression Test - Evaluating individual cylinders.

A running (dynamic) compression test, confirms how efficiently each cylinder is pulling air in. Next, 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:

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.

Compression Tester
Compression Tester

NOTE: You could use a Lab Scope and Pressure Transducer to see the results. But, we are going to perform this test, using, a basic engine compression testing gauge.

Consequently, 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

So, the first thing you need to do is, create a chart; similar to this example, to record your test readings.

CYL CRANKING-(STATIC) IDLE SNAP
1 ? ? ?
2 ? ? ?
3 ? ? ?
4 ? ? ?

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.
  • Holes in pistons.

So, 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).

So, 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.
Engine Compression Gauge Showing High Compression
Engine Compression Gauge Showing High Compression
Engine Compression Gauge Showing Low Compression
Engine Compression Gauge Showing Low Compression

A Low, Snap Test Reading

If a snap reading is low (much less than 80% of cranking compression); look for air intake problems.

Such as:

  • 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. So, look for problems, on the exhaust side.

Such as:

  • 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.

Such as:

NOTE: Consequently, a cylinder leak down test will narrow down, where the leakage is coming from.

Conclusion

So, as you can see, information gathered from a running (dynamic) compression test; can be very helpful when diagnosing, engine misfires issues. And, another great test to, help solve engine problems.

Thank You !