RUNNING COMPRESSION TEST
This is a summary of the responses to a question about a "Dynamic Compression Test" sent out via the i-ATN e-mail list and posted on Compuserve's "For Techs Only" forum. It seemed to ring a bell with the most techs as a "running compression test," so I will use that name here. Call it what you will, this test is an accurate if slightly esoteric and time-consuming test of cylinder breathing. It is in fact recommended by Detroit Diesel instead of a traditional static compression test, it is included as part of Delmar's ATTP program, and several instructors use it as part of their state emission training programs. Doing this may save your engine from dying or having to donate car.
HOW TO PERFORM A RUNNING COMPRESSION TEST
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 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.
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 this: CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 125 Cyl 2 175 80 130 Cyl 3 160 75 120 Cyl 4 160 80 125
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.
EXAMPLE 1 - RESTRICTED INTAKE CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 80 If Snap reading is low (much less than 80% cranking compression), look for restricted intake air- severely carboned intake valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the runners. (Toyota, Vortec etc. with variable runner length) Comparing measurements between cylinders is important.
EXAMPLE 2 - RESTRICTED EXHAUST CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 180 If snap measurements are significantly higher than 80% of cranking measurements, look for restricted exhaust on that cylinder-such as worn exhaust cam lobe, or collapsed lifter. Or, if they are all high, look for a clogged cat converter.
WHAT IS GOING ON?
When you do a normal compression test, you are checking cylinder sealing, not cylinder breathing. When you check engine vacuum at the manifold, you are looking at the breathing of the entire engine, by checking vacuum at a common (plenum) source. You aren't testing a specific cylinder. This test looks at the breathing of an individual cylinder.
Say the engine is running at 18 inches vacuum. Atmospheric pressure is about 30 inches, so the difference (30 inches - 18 inches = 12 inches) is what the engine is sucking in. 12 inches mercury is equivalent to about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you should get 6 x 8 = 48 psi pressure if all the air makes it into the cylinder and then gets pushed out. So your idle reading on running compression is about 50 psi.
When you snap the throttle, the manifold vacuum drops, so the absolute air pressure going into the cylinder increases.
In fact, you can do running compression tests at various constant manifold vacuum readings (by brake-torqueing the engine momentarily), and the running compression should roughly correspond to the manifold vacuum. For example, at 10 inches vacuum, engine should be breathing in about 10 psi air pressure, so you should see a running compression reading of about 80 psi (at 8 to 1 compression ratio).
If one cylinder reads low running compression compared to the rest it means that the air didn't make it in. If one cylinder reads high, the air didn't make it out (and the next pulse of air raised the pressure).
Many thanks to the people who responded through both Compuserve and i-ATN. Special thanks go to Bruce Amacker, Bob Cammarano, Chris Chesney, Troy Croskey, Don Hazlett, Rick Jensen, Greg McConiga, David Palin, & Thomas ??(whose last name I seem to have lost)
Rick Greenspan College of Alameda firstname.lastname@example.org
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