Valve difficulties...
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Joined: Apr 2006
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From: Kansas City, KS
Valve difficulties...
OK boys. Here we go again. It's a 351M in a 79 F150. I've got a miss in cylinder six. It does have spark. Per your suggestions, I bought a compression tester and sure enough, number six registers a whopping zero. Number two is also low (60 lbs) but the rest look OK. (100 to 110 lbs.) I removed the valve cover and cannot see anything drastically wrong. i.e. no broken springs. Now what? I'm assuming that the exhaust valve is the issue here, but I don't know how to tell. I find it hard to believe that the rings are bad in one cylinder. Any ideas?
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Back off the rockers on #6, and put air pressure into the cylinder. Look for air flow out of the exhaust (exhaust valve), intake manifold with carb open (intake valve), or crankcase (ring or piston).
I am betting on a cracked piston or broken ring, because even bad valves seal sometimes. Did you have any oil burning?
I am betting on a cracked piston or broken ring, because even bad valves seal sometimes. Did you have any oil burning?
Posting Guru
Joined: Mar 2005
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From: western montana
I recently took one apart and had 3 compression rings break when removing pistons- no real lip to speak of but had 80 or less in all 3. No pressure seems interesting to me...but most things i can't explain are.
So best case scenario is rebuild heads...most likely engine needs it...thats my luck...the luck of the unirish.
So best case scenario is rebuild heads...most likely engine needs it...thats my luck...the luck of the unirish.
Elder User
Joined: Feb 2002
Posts: 987
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From: East Bend, Carolina, U.S
A cylinder can also have low to zero compression if the piston is damaged (usually accompanies cracked rings). If only one comporession ring is cracked on a piston, then it can still maintain a fair amount of compression. Now if the same scenario is taking place with a damaged piston (ring lands cracked through to the inside of a piston) then you will see a huge loss in compression.
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Joined: Feb 2002
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From: East Bend, Carolina, U.S
[QUOTE=grclark351]a severely damaged or holed piston will be pretty obvious if you remove the oil fill cap while the engine is running, it'll be blowing air in and out.[/QUOTE
Yep, that's at least one thing tcbofade needs to check for.
Yep, that's at least one thing tcbofade needs to check for.
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Posting Legend
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Originally Posted by grclark351
a severely damaged or holed piston will be pretty obvious if you remove the oil fill cap while the engine is running, it'll be blowing air in and out.
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Joined: Apr 2006
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From: Kansas City, KS
OK, well I guess the good news is that it didn't look like a steam engine. lol. I've got a buddy with an air compressor and we'll chase the air flow...hopefully tonite. Otherwise I'll have to wait for the weekend. I realize that either way I end up pulling the head, but if I've got a bad piston and or ring, I want to know that I'm looking at a complete rebuild before I start.
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