What is the correct way to do a compression test?


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By oldcarmark - 15 Years Ago
Should have asked BEFORE I did one today.Motor warmed up.Choke and throttle wide open. 3-4 compression strokes comes up at 130.Is 3-4 sufficient for an accurate reading? If I let it go 5-6 its about 150. Just wanted to verify that I did not make a mistake with the timing gears when I assembled this motor.I think they are correct with this high a compression reading.
By rick55 - 15 Years Ago
Technically 3-4 sould be sufficient, but it seems in the real world more like 5-6.

150 is pretty good.

The important thing is that they are all even, no more than 20 between readings.

Finally give a good squirt of oil from an oil can into the cylinder with low readings and see if this improves - it it does its rings, if not its valves.

Regards

By Ted - 15 Years Ago

You’ll want to block the throttle in the wide open position while performing the cranking compression test and do the test with all the spark plugs removed.  With all the spark plugs out, up to six hits on the gauge may be what it takes to get a maximum reading.  Do not perform the test on a heated engine as this can damage the gauge.  If doing the compression check with only one plug out at a time, then a lower reading at each cylinder will be obtained.  As a rule of thumb, the difference between the lowest and highest readings should not exceed ten percent.  If the cranking compression is 150psi, then 15 lb variance between the highest and lowest is all that’s to be expected and be within tolerance.  Do the first test with no oil in the cylinders and if the numbers are low, then repeat the test with oil and take note how much each reading increases over the previous readings.

By Speedbump - 15 Years Ago
Another cool thing to do, if you have it, is hook a battery charger up on low.  That keeps the battery at the same cranking strength for all cylinders.  Also, in a healthy engine, the "first puff" of the cylinder on the gauge should be around 40-50 percent of the total or more, with successive puffs being progressively smaller.  If it's not, and a wet or oil squirt test makes it happen it's a sure sign of a ring problem.
By PF Arcand - 15 Years Ago
Further, is the ignition supposed to be disabled in some way, or grounded when testing?..
By Speedbump - 15 Years Ago
The safest way, especially with electronic, is to disconnect the coil + on the primary side and make sure it can't go to ground when the key is on, like with a piece of rubber hose or taping it securely out of the way.  If that's a big hassle, there is usually no problem with grounding the high tension cable from the coil to ground.  Don't take it out of the distrubutor cap and let it hang.  It will look for a ground, sometimes to the detriment of the system.  Smile
By oldcarmark - 15 Years Ago
I disconnected the bypass wire on the solenoid(the one that supplies 12 volts initially to start the motor).I use a remote starter button clipped onto the battery + and the "s" terminal on on the solenoid.Never used the ignition switch at all.The ignition system is not energized at all.