Propper Compression Test
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Freshman User
Joined: Nov 2006
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Propper Compression Test
My question is how to do a propper compression test on my 302. Suspect I might have a dead cylinder. Have heard in the past that the wires to the injectors need to be disconected. Is this correct? Also if I have a busted rocker arm how can I find out without pulling the valve covers? Thanks for any help.
Cargo Master
Joined: May 2007
Posts: 2,071
Likes: 35
From: British Columbia
Propper (sic) compression test
The spark plugs should all be removed, and the ignition and fuel systems disabled, typically by removing the relays for them. The throttle is held wide open for the test. The compression tester is installed in the first spark plug hole, the engine cranked for either 3 revolutions or until the gauge reads no higher (there are different schools of thought on which is better, I usually choose the latter) and the reading recorded. The remaining cylinders are tested similarly.
Screw in type testers are better than the type with a rubber plug on the end that you push against the plug hole with, and the testing should be done on a warm engine.
Screw in type testers are better than the type with a rubber plug on the end that you push against the plug hole with, and the testing should be done on a warm engine.
Cargo Master
Joined: Dec 2004
Posts: 2,232
Likes: 3
From: NE Texas
Although most books say to wire the throttle open, you can do a meaningful compression test without that step.
Remove all plugs, disconnect injector harness, attach or hold gauge and spin it over until the needle is as high as it will get. Right the numbers down for all 8 cylinders. They should be within 10% of each other. If there is a low one, squirt oil in the cylinder and retest that cylinder. If that cylinders compression comes up, the problem is cylinder related and if not it is valve or head gasket related.
If you do indeed have a low cylinder, you can then determine a lot more with a cylinder leak down test. Apply the pressure to the low cylinder and listen for where the air is escaping.
If heard through the crankcase breather or filler, it is cylinder related. If heard through the tailpipe it is exhaust valve related. If heard through the throttle body it is intake valve related.
Hope this helps.
Remove all plugs, disconnect injector harness, attach or hold gauge and spin it over until the needle is as high as it will get. Right the numbers down for all 8 cylinders. They should be within 10% of each other. If there is a low one, squirt oil in the cylinder and retest that cylinder. If that cylinders compression comes up, the problem is cylinder related and if not it is valve or head gasket related.
If you do indeed have a low cylinder, you can then determine a lot more with a cylinder leak down test. Apply the pressure to the low cylinder and listen for where the air is escaping.
If heard through the crankcase breather or filler, it is cylinder related. If heard through the tailpipe it is exhaust valve related. If heard through the throttle body it is intake valve related.
Hope this helps.
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Stingray42
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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Oct 24, 2003 06:27 PM
