Compression test
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From: Lockport, NY USA
Compression test
I have i 351W that i just picked up. It was supposedly rebuilt 500 miles ago but i want to check it out before i drop it in. I have never compression tested an engine that is not in a vehicle. Can i just put a ratchet on the crank and turn it over a few times? How long should the engine hold the compression for?
-Jon
-Jon
Senior Member
Joined: Aug 2001
Posts: 378
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From: Houston, Texas
Nope, that won't work because I don't think you can turn the engine fast enough. Remember, you get a compression stroke only every other revolution. To do a compression test, do the following:
a) remove the sparkplugs from all the cylinders
b) make sure your engine is securely fastened to the stand and make sure the stand is fastened securely to the floor or a wall.
c) screw the compression tester into a cylinder
d) hold the throttle butterfly valve at wide open throttle
e) with the starter motor, make at least 4 complete revolutions of the motor. Or, in the alternative, crank until the compression meter doesn't jump up any more.
If after you complete the test, you have one or more cylinders that are 15 or more lbs below the rest, then you can do a wet test on these cylinders. Put 1 or 2 tablespoons of oil in the cylinder. Then, do the compression test as described above. If you get higher compression, the chances are good that the rings are bad.
As far as your question, "how long should it hold compression?" Well, this is not possible to do with a standard compression tester, because obviously the valves open up and you have zero pressure. The compression tester gauge 'remembers' the highest compression obtained. Besides, if you can build the required compression with the starter motor, you know that you will have sufficient compression when running at normal motor speeds.
If you want to check this (and it is really only necessary if you got bad compression readings from the standard compression test), then you need to get a "leak down" tester. This is a device that has two gauges and uses compressed air.
To check a cylinder, it must be at top dead center with both valves closed. Screw in the leak down tester, and apply compressed air. The first gauge will show you the PSI in the cylinder. The second gauge will show you the leak down rate. If you can hear air coming out the tailpipe, it is your exhaust valves, if the noise emanates from the intake, it is your intake valves, and it you hear noise through the oil breather, it is your rings. If you can't tell, it is probably the head gasket. If the engine is in the truck when you do this, you could take off the radiator cap and see if you can see bubbles.
a) remove the sparkplugs from all the cylinders
b) make sure your engine is securely fastened to the stand and make sure the stand is fastened securely to the floor or a wall.
c) screw the compression tester into a cylinder
d) hold the throttle butterfly valve at wide open throttle
e) with the starter motor, make at least 4 complete revolutions of the motor. Or, in the alternative, crank until the compression meter doesn't jump up any more.
If after you complete the test, you have one or more cylinders that are 15 or more lbs below the rest, then you can do a wet test on these cylinders. Put 1 or 2 tablespoons of oil in the cylinder. Then, do the compression test as described above. If you get higher compression, the chances are good that the rings are bad.
As far as your question, "how long should it hold compression?" Well, this is not possible to do with a standard compression tester, because obviously the valves open up and you have zero pressure. The compression tester gauge 'remembers' the highest compression obtained. Besides, if you can build the required compression with the starter motor, you know that you will have sufficient compression when running at normal motor speeds.
If you want to check this (and it is really only necessary if you got bad compression readings from the standard compression test), then you need to get a "leak down" tester. This is a device that has two gauges and uses compressed air.
To check a cylinder, it must be at top dead center with both valves closed. Screw in the leak down tester, and apply compressed air. The first gauge will show you the PSI in the cylinder. The second gauge will show you the leak down rate. If you can hear air coming out the tailpipe, it is your exhaust valves, if the noise emanates from the intake, it is your intake valves, and it you hear noise through the oil breather, it is your rings. If you can't tell, it is probably the head gasket. If the engine is in the truck when you do this, you could take off the radiator cap and see if you can see bubbles.
Last edited by BeastRider; Oct 24, 2001 at 12:00 PM.
