Running Compression Testing

Jim Mobile Technician Southampton, Pennsylvania Posted   Latest   Edited  

Now that Running Compression testing has become such a main stream procedure, I’ve been asked more than once, “Isn’t that actually an old test that has been around forever”?

The answer is Yes and No. I actually first learned and performed this test back in Detroit Diesel school when I worked for a construction equipment company in the 60’s. The next time that I remember it being taught and talked about was at one of the Great Linder Tech Training events of the past.

In my opinion, nobody should EVER do any test procedure unless you understand why you are doing that test and what to do with the results.

This test is actually about Cylinder Breathing, and when we connect a vacuum gauge up to an engine, what you are actually checking is the Breathing of the ENTIRE engine. Think about it, after starting the engine, the vacuum gauge should have been pulled down to 18 to 21” hg. But how it did it and why it did it is because all of the engine’s cylinders did their performed duty. When I tried to teach this to my Post-Secondary Advanced Engine Performance class, I would signal out six of the students sitting in a row. I would indicate the first student was cylinder #1, what he did when the engine started was pulled the gauge down to that 18 to 21” hg of vacuum, But why did the gauge stay there ??? Because the other cylinders in turn did the same great job.I would then point out that the 4th student in line fell asleep, what would happen to the pointer on the gauge when it was his turn to fire ??? The needle would start back toward the zero” hg but the next cylinder working correctly would again pull the pointer back to that 18 to 21”hg. But you would NOT know which cylinder was the problem.

A Running Compression test with a compression gauge is checking individual cylinder breathing. With the engine running, I prefer leaving the Schrader valve in the tester and “Burping” the gauge after a few seconds of running. The following waveform will be the output you will get: 

( I’m using my Lab Scope and Pressure Transducer so you can see the results, But this can be performed using the basic compression gauge also). 

As you can see at about the 3.5 second point, I snapped the throttle to see how that cylinder INHALED and EXHALED. A good cylinder inhaling and exhaling will produce approximately 75 to 80% of Cranking Compression. If the Snapped reading is lower then expected, then is an issue with the cylinder INHALING. If the Snapped reading is higher than expected, then is an issue on the exhaust side of that cylinder. If you do receive a bad reading, check other cylinders to see if you have a Common problem, like an intake restriction or a clogged Catalytic Converter.

  1. Start with a normal ("static") compression test. To eliminate rings, valves, holes in pistons, that sort of things. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm.
  2. Put all spark plugs but one back in. Ground that plug wire or remove the COP coil to prevent module damage. Disconnect that injector on a port fuel system.
  3. Put your compression tester into the empty hole. The test can be done without a Shrader valve, but most people recommended leaving the valve in the gauge and "burping" the gauge every 5-6 "puffs".
  4. Start the engine and take a reading. Write it down.
  5. Now goose the throttle for a "snap acceleration" reading, reading should rise. Write it down NOTE: Don't use the gas pedal for this snap acceleration. The idea is to manually open then close throttle as fast as possible while without speeding up the engine. This forces the engine to take a "gulp" of air. Note – This could be s challenge with Electronic Throttle Control systems.
  6. Now write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe 2-3 good ones. Make a chart like the one inline below: (I think you can download this and load it into excel from here)
  7. ANALYSIS: Running compression at idle should be 50-75 psi (about half cranking compression). Snap throttle compression should be about 80% of cranking compression.
Geoff Diagnostician
Lahaina, Hawaii

Gonna save that one, Jim. It's never been taught in any class I have attended that I can remember. Obviously I have never done it. Nobody has a transducer out here, either, far too expensive a tool for all the POS beaters out here. We have one "customer" driving around with four good cylinders in a V6 Ranger. They all seem to have money for beer, drugs, and gas, but not a new engine.

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