No Compression!
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Freshman User
Joined: Sep 2006
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No Compression!
I have had a dead cylinder on my 1985 f250 6.9l truck and was not sure which one it was so I did a compression test today. All 7 cylinders were 400 lbs plus or minus 5 lbs. But all very close. The passengers rear cylinder has no compression. Yes it has "0" lbs of compression. I removed the test guage and put my finger over the hole and there was no pressure at all while cranking the engine. What Ideas do you have for this! My only guess is it dropped a valve and knocked a hole in the piston. I dont know what else could cause this cylinder to have 0 compression.
Elder User
Joined: Aug 2005
Posts: 566
Likes: 3
From: New Jersey
Probably a burned valve. I've pulled engines apart that had a D-shaped exhaust valve. It gets that bad because once there is an edge that can burn, it continues to burn until a significant portion of the valve is gone.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 15
From: Nutter Fort, WV
Oil sucked into the CDR and then on into the intake tends to be burned mostly in the rear cylinders.
Since motor oil has more BTU's than diesel does it makes the fire hotter in cylinder 7 and 8.
P.S. Your problem is in cylinder 7.
Since motor oil has more BTU's than diesel does it makes the fire hotter in cylinder 7 and 8.
P.S. Your problem is in cylinder 7.
Posting Guru
Joined: Mar 2005
Posts: 1,701
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From: KY
Pll the valve cover off and them the rocker assembly for that cylinder. Put shop air into the glowplug hole with a rubber tipped air gun. That will tell the story.
I knew an old man, when I was a kid, that lost a piston in a 300/6. He drove a block of wood in that hole and ran the engine on 5 cylinders for along time. You probably don't want to do that but I saw it done.
I knew an old man, when I was a kid, that lost a piston in a 300/6. He drove a block of wood in that hole and ran the engine on 5 cylinders for along time. You probably don't want to do that but I saw it done.
Post Fiend
Joined: Nov 2004
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From: Manitoba
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Posting Guru
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From: KY
Originally Posted by PLC7.3
Catfish your telling your age........ you can't get that good wood anymore.......
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Elder User
Joined: Aug 2005
Posts: 566
Likes: 3
From: New Jersey
If you do indeed have a burned valve, then you're looking at pulling the heads and getting a valve job. The air nozzle compression check is a great idea, and will definitely give you a good indication of where the failure is, but no matter which valve is dead, you're looking at pulling the head to get it fixed.
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