Help Reading/Understanding Compression Testing Results
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Junior User
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Help Reading/Understanding Compression Testing Results
Last night I hooked up my compression gauge to get some sense of what the compression was on just one of the cylinders. We tested the first, front-most cylinder on the Driver's side.
In the middle of it all, I realized I didn't know what I was doing. Because the pressure was different when the vehicle was first started compared to what was read after it had run for a few seconds.
And in the 2nd test, the pressure readings were much lower, which is the opposite of what I would have expected.
And finally, I have no idea what a good or bad pressure might look like.
In the middle of it all, I realized I didn't know what I was doing. Because the pressure was different when the vehicle was first started compared to what was read after it had run for a few seconds.
And in the 2nd test, the pressure readings were much lower, which is the opposite of what I would have expected.
And finally, I have no idea what a good or bad pressure might look like.
Hotshot
Joined: Jul 2005
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Tyrone, I get the idea that you are STARTING the motor to get the compression readings ? That might give false and HIGH readings.. Actual readings are taken while cranking the motor with NO FUEL and NO SPARK available. You normally crank for 5 seconds to get several pulses into the pressure gauge, then it stops going up and that is your MAX. Like mentioned before, the pressure is THEORETICALLY atmospheric pressure ( 14.7 psi) times the compression ration ( normally 8-9).
Fleet Owner
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Lead Driver
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Proper testing is done (with all plugs removed) and throttle opened at least half way.
Otherwise you cranking cylinders under compression and excessive effects of battery drain as each cylinder it tested in turn.
Good luck.
Otherwise you cranking cylinders under compression and excessive effects of battery drain as each cylinder it tested in turn.
Good luck.
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Junior User
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Tyrone, I get the idea that you are STARTING the motor to get the compression readings ? That might give false and HIGH readings.. Actual readings are taken while cranking the motor with NO FUEL and NO SPARK available. You normally crank for 5 seconds to get several pulses into the pressure gauge, then it stops going up and that is your MAX. Like mentioned before, the pressure is THEORETICALLY atmospheric pressure ( 14.7 psi) times the compression ration ( normally 8-9).
Here's a pretty good "how to"...
https://www.2carpros.com/articles/ho...ne-compression
https://www.2carpros.com/articles/ho...ne-compression
Meh. Sounds like a lot of trouble. I just replaced all the spark plugs and spark plug wires. That would have been the right time to do compression testing. Grr...
Hotshot
Joined: Jul 2005
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You can test ONE cylinder if you want with the rest of the plugs in... just don't START the motor. Crank for 5 seconds and watch the needle jump 20 -40 -60 -80 -100 ... then it will stop somewhere around 120-140 psi... Then stop cranking. You have to disable the fuel somehow.. fuse or inertia switch.
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Junior User
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You can test ONE cylinder if you want with the rest of the plugs in... just don't START the motor. Crank for 5 seconds and watch the needle jump 20 -40 -60 -80 -100 ... then it will stop somewhere around 120-140 psi... Then stop cranking. You have to disable the fuel somehow.. fuse or inertia switch.
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Hotshot
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Lowest should be within 75% of the highest assuming that the highest is with the typical range (above). It's that simple.
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Hotshot
Joined: Jul 2005
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Tyrone, I don't know your "spec". but most engines have about 9 : 1 compression ratio. Atmospheric pressure is 14.7 psi so 14.7 x 9 = about 130 psi..... if they all read 120 your OK....... if they all read 135 psi your OK... problem is if 3 of them read 130 psi and one reads 90 psi....... that's a problem. .......... hopefully nothing is below 110.
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