Correct way to test compression?
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Senior User
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From: Sonoma, CA
Correct way to test compression?
What is the best way to do a compression test? All plugs out? Engine warm or cold? Throttle open or closed? I want to check out the new truck I am getting, but am not sure I know the correct method.
Thanks,
Mike
Thanks,
Mike
Post Fiend
Joined: Mar 2005
Posts: 8,542
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From: Louisiana
Do you have a compression testing kit? I got one from autozone a while back, before I rebuilt my 302, and I just followed the directions, I want to say I just took out one spark plug at a time, but can't remember for the life of me right now.
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Welcome to the site 
This question should have been asked in the general automotive discussion forum as it isn't really 73-79 truck related.
Before it gets moved!
Plugs out ! to allow possible cylinder leak across head to other cylinder.
First DRY Test then Wet Test
only 3 complete cranks to build pressure as this will allow consistency per cylinder.
Cold is best as ALL the parts will be the same temp and will prove to give a better indication of the condition of the engine.
If the engine is heated some parts expand differently
Carb left in normal position.
This question should have been asked in the general automotive discussion forum as it isn't really 73-79 truck related.
Before it gets moved!
Plugs out ! to allow possible cylinder leak across head to other cylinder.
First DRY Test then Wet Test
only 3 complete cranks to build pressure as this will allow consistency per cylinder.
Cold is best as ALL the parts will be the same temp and will prove to give a better indication of the condition of the engine.
If the engine is heated some parts expand differently
Carb left in normal position.
Last edited by Mil1ion; Feb 18, 2006 at 12:56 PM.
Hotshot
Joined: Dec 2003
Posts: 18,423
Likes: 9
From: iowa
Wrong, carb should be in open position, cold will give you the lower compression to tell you if the rings are still seating decently (warm test will have the pistons expanded and will raise the compression test and make it look better but cold will tell you more accuratly what the rings are doing.) don't bother with the wet test unless you have one or more cyl that are way out of line, then a wet test will tell you wether it's the rings or something top side. plugs in or out isn't really a big deal although I test with them out forst because you can get faster crank speed, and second because it takes less amperage to turn it over that way so your not running the battery down so bad and thus spinning hte engine at a slower speed on the last cyl than you were on the first cyl.
Senior User
Joined: Sep 2001
Posts: 350
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From: Friendswood TX
My 76 Ford Shop Manual sez plugs out, carb open, warm motor, 5 cranks, lowest pressure no less than 75% of highest reading, good battery (I put mine on a charger now and then during a compression check a couple of weeks ago), tablespoon of oil in the spark plug hole of the out of spec cylinder to check for bad rings.
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From: stuart fl
sorry wrong post onthis subject
Last edited by 7702fordboy; Feb 18, 2006 at 08:07 PM. Reason: wrong post
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Cargo Master
Joined: Apr 2001
Posts: 2,364
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From: Eustis FL
I'm with the warm test. You want to know your compression under normal operating conditions (warm).
If engine is dead, you have no choice but to test cold. (like purchasing a non-running vehicle, you want to diagnose the problem)
Throttle fully open will ensure enough air gets into the cylinder for the test.
All plugs out is best, but not practical for dual-spark plug engines. (duh! haha)
With EFI, pull fuel pump fuse.
And as mentioned, wet test only needed when a cylinder is low. Even if within 75% of other cylinders, a wet test on a 'lower' reading cylinder will help diagnose a ring or valve problem.
I like the 5 crank procedure. 3 just doesn't cut it for me. I have gotten an additional 10-20 psi on those last two cranks.
Mark
If engine is dead, you have no choice but to test cold. (like purchasing a non-running vehicle, you want to diagnose the problem)
Throttle fully open will ensure enough air gets into the cylinder for the test.
All plugs out is best, but not practical for dual-spark plug engines. (duh! haha)
With EFI, pull fuel pump fuse.
And as mentioned, wet test only needed when a cylinder is low. Even if within 75% of other cylinders, a wet test on a 'lower' reading cylinder will help diagnose a ring or valve problem.
I like the 5 crank procedure. 3 just doesn't cut it for me. I have gotten an additional 10-20 psi on those last two cranks.
Mark
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Laughing Gas
Joined: Mar 2004
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From: Maple Ridge B.C. Canada
Originally Posted by monsterbaby
Wrong, carb should be in open position, cold will give you the lower compression to tell you if the rings are still seating decently (warm test will have the pistons expanded and will raise the compression test and make it look better but cold will tell you more accuratly what the rings are doing.) don't bother with the wet test unless you have one or more cyl that are way out of line, then a wet test will tell you wether it's the rings or something top side. plugs in or out isn't really a big deal although I test with them out forst because you can get faster crank speed, and second because it takes less amperage to turn it over that way so your not running the battery down so bad and thus spinning hte engine at a slower speed on the last cyl than you were on the first cyl.
Hotshot
Joined: Dec 2003
Posts: 18,423
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From: iowa
Originally Posted by GLR
Carb is in wide open throttle position to keep fuel from entering cylinders, dont forget to ground coil wire to dist or disconnect primary wire at coil, to keep from getting shocked.
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