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Old 06-13-2010, 09:04 PM   #1
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Default Lost compression??

Well, I had a wild hair to do a compression test on my Bonneville last night and I am severely disappointed with my results.

I only tested the front 3 cylinders (I hate shoving the tester in the back 3). Cylinders 1, 3, and 5 I think.

Cylinder 1 (or 2): 95 psi
Cylinder 3 (or 4): 110 psi
Cylinder 5 (or 6): 130 psi.

If I'm not mistaken, 130 is normal. The closer I get to the tranny, the more pressure is lost. What could cause this, besides an old and work engine?
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Old 06-13-2010, 09:08 PM   #2
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How many miles on the motor?
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Old 06-13-2010, 09:09 PM   #3
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I assume these were tested with the throttle wide open correct? I had the same situation for the longest time before I realized my shortcoming.
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Old 06-13-2010, 09:19 PM   #4
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I don't know the engine miles, but the chassis says 307,600. Maybe original motor, maybe not. I has NO idea.

The throttle wasn't open at all. Just unhooked the coilpack ECM and cranked with the tester plugged in. Does the throttle affect it?
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Old 06-13-2010, 09:22 PM   #5
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I think it does, because it is trying to draw air through a mostly closed throttle plate. Experts? Throttle open or closed?
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Old 06-13-2010, 10:48 PM   #6
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Throttle needs to be full open. Also test with a warmed up engine and get 5 to 10 good cranks on for a good test. Test all cylinders and if you have more than a 10% difference between the highest and the lowest reading, you are getting in the area to where you may need t look at a rebuilt motor.
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Old 06-13-2010, 11:27 PM   #7
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The amount of compression you have is not important unless it'* really low, it'* a balance between them all, 10% max difference. Here'* a write up on doing a compression test.
https://www.gmforum.com/trouble-shooting-test-procedures-141/compression-test-284387/
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Old 06-13-2010, 11:46 PM   #8
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as stated above

warm engine, throttle wide open.

balance between cyls is more important then the actual number
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Old 06-14-2010, 03:11 AM   #9
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Thanks guys. I'll do it again with WOT here soon. It was warm last time I did it, and I'll warm it again. That makes it a pain in the *hands* cuz the exhaust is just right there with the spark plugs!
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Old 06-15-2010, 02:00 PM   #10
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Just as another note there are two ways to do the test...first off all the responses are correct lol. u can also do a test where ya squirt a bit of oil in the empty spark plug hole and then crank it over if it holds your rings are good if it bleeds down they re getting worn if i recall correctly...
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