Compression test help
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Compression test help
working on 1968 mustang inline 6 200 and found low pressure in cylinder 1 and pulled the head. Cylinders looked clean, but carb had been sucking oil out of the head into the air cleaner and I thought maybe it was the valve seals. Car set without running for 30yr, but son has been driving it for a few months. We went ahead and had a valve job done and put it back together and retested with these results:
dry test
cyl 1 = 50-60psi
2-6 = 150-160psi
wet test 2tbsp 40w oil
cyl 1 80-90 psi
Does anyone know if this points more to rings or maybe lifter or ?
dry test
cyl 1 = 50-60psi
2-6 = 150-160psi
wet test 2tbsp 40w oil
cyl 1 80-90 psi
Does anyone know if this points more to rings or maybe lifter or ?
Moderator
Joined: Dec 2007
Posts: 7,986
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From: Boise, Idaho
If it's stuck rings, you can try swapping out a quart of oil with some Marvel's Mystery Oil. Alternatively, a quart of ATF can do wonders for cleaning things out.
Give this a read:
Stuck Piston Rings: snake-oil suggestions please | The H.A.M.B.
Give this a read:
Stuck Piston Rings: snake-oil suggestions please | The H.A.M.B.
Fleet Mechanic
Joined: Nov 2002
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It may be stuck rings, in which case I would drive it a while with the aforementioned additives to try to un-stick the rings. 50-60 PSI is enough to fire that cylinder. It may drop out at idle giving you a mis-fire, but driving it down the road it will probably make some power in that hole.
Alternatively, it could have a broken ring or piston ring land. If the problem does not rectify itself with further driving and a crankcase purge I would suspect that, especially if the cranking compression remains unchanged, or further deteriorates.. Good luck
Alternatively, it could have a broken ring or piston ring land. If the problem does not rectify itself with further driving and a crankcase purge I would suspect that, especially if the cranking compression remains unchanged, or further deteriorates.. Good luck
Fleet Mechanic
Joined: Sep 2014
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From: NE Texas
From what I remember, if the oil increases the compression appreciably you almost certainly have a ring issue. MMO, ATF, lots of things are touted for unsticking stuck rings. One precaution - if you can obtain a borescope or suchlike to look inside at the cylinder walls, I would recommend it as soon as possible - a broken ring would have the same symptoms and if you can avoid getting the cylinder walls too scored up, your rebuild will be a lot faster and cheaper.
Fleet Mechanic
Joined: Nov 2002
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I almost posted the same caveat about a broken ring damaging the cylinder wall except that I disassembled my lawn tractor engine last year only to find a broken top ring. After what must have been several decades of operation like that the bore was fine. I guess it depends on how lucky you feel.
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Thanks for the replies. We had the head off last week when doing the valve job and the cylinders were very smooth. I may try to drive it with some mystery oil; but it barely runs right now.
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Fleet Mechanic
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One weak cylinder should not make it barely run. I would look elsewhere for issues.
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update: Pulled the valve cover and tried to work the lifter with the pushrod and I could not feel it move. I tapped the top of the pushrod with a hammer and put the valve cover back on. It now cranks and runs pretty good; will try the mm oil anyway since it seemed to have a lifter issue. Thanks for all the replies.
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