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no compression on #8

phred

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Aug 25, 2006
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Earth
Running compression test on Mac's 302 all cylinders are running 115 to 125 except #8. truck runs fine with out a skip but it does smoke a bit. Needs some thoughts. I'm thinking compression ring is gone. There is some oil on the plug.
thanks
 

d.c.parris

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Aug 26, 2005
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Loc.
cumming ga.
pull the spark plug and pour about 2 table spoons of engine oil into the cyl. then check compression if it comes up then you most likely have a ring problem if it dont come up you have a valve problem, a hole in the piston or blown head gasket. good luck don
 

ryan_289

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Apr 24, 2007
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Loc.
Russellville, AR
I saw this trick on Horsepower tv the other day. Take an old spark plug and knock the porcelin out of it and cut off the spark tip. Then weld an air chuck to the plug and you have a basic leak down test. turn the engine over to TDC on #8 and screw in the modified spark plug. Hook up the air. If you get air out of the breather its bad rings, open the throttle valve, if there is air escaping out of the carb its bad intake valve, if there is air coming out of the exhaust its a bad exhaust valve.
 
OP
OP
phred

phred

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Earth
Ok,
Did a wet test and still 0 compression. Looks like the #8 exhaust valve stud has backed out of the head about 1/4". I'm thinking the valve is hanging open. Think is time for a head job.
 

Mac2Night

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Aug 10, 2005
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2,199
Brett,

Thanks a bunch for your help today, it was nice having another knowledgeable EB owner to hang out with and you seem to have me on the right track. :)

Anyone else out there help confirm that I could have good compression (120-125 psi) on (7) cylinders and zero (0 psi) on #8 because of an exhaust or intake valve staying open??????

Thanks,
Mac

Ok,
Did a wet test and still 0 compression. Looks like the #8 exhaust valve stud has backed out of the head about 1/4". I'm thinking the valve is hanging open. Think is time for a head job.
 

Viperwolf1

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electron whisperer
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It sounds reasonable. Your gonna have to pull that head off anyway even it's a broken piston.
 

Pa PITT

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Stephenville TEXAS
I say it sounds like you found your problem...as for making the item out of the old plug ...I've done the same thing except I used ONE OF THOSE things called a non-fouler ..these items have your spark plug thread to screw into the block and then your supposed to screw a new hot plug into them I've found their easy to drop a airline nipple into them and weld up or JB-WELD..
 

blubuckaroo

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Jun 11, 2007
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Loc.
Ridgefield WA
I say it sounds like you found your problem...as for making the item out of the old plug ...I've done the same thing except I used ONE OF THOSE things called a non-fouler ..these items have your spark plug thread to screw into the block and then your supposed to screw a new hot plug into them I've found their easy to drop a airline nipple into them and weld up or JB-WELD..

PAPITT,
Man, we have to get together sometime to share our JB Weld stories!
 

Broncobowsher

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Messages
34,833
I have used a compression tester (with the special low tension schrader valve removed) and the quick connect hooled to an air line to find a burned intake valve before.

Personally I suspect a broken ring on a worn engine.
 

Mac2Night

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Aug 10, 2005
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Broncobowsher,

The engine started acting up only when the rocker arm over the #8 cyl. came loose (turned sideways on the stud) and I realized the stud had pulled up about 1/8 -1/4 in. I shimmed the rocker arm back on the stud as a temp fix. Since then, the #8 plug has stayed oil drenched (clean oil, not fouled) and the compression test showed no compression on that cylinder. Wet testing the cylinder didn't produce any compression and when we pulled the valve cover and observed the valves with the engine turning over, the shimmed rocker arm didn't move as much as the others did, leading us to think that this valve is not open and closing correctly, hence an exhaust and/or intake leak.

Do you still think it could be a ring on this one piston?




I have used a compression tester (with the special low tension schrader valve removed) and the quick connect hooled to an air line to find a burned intake valve before.

Personally I suspect a broken ring on a worn engine.
 

Pedestrian

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Sep 10, 2008
Messages
2,299
I have used a compression tester (with the special low tension schrader valve removed) and the quick connect hooled to an air line to find a burned intake valve before.

x2 Way easier than tearing a spark plug apart and adapting it.
Just don't lose the schrader and replace it with a regular schrader, everything looks like it needs a rebuild after that ;D
 

Broncobowsher

Total hack
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Jun 4, 2002
Messages
34,833
It is possible that you overshimmed the rocker, holding the valve open. But I would suspect other issues if that were the case. Something you could do, take the rockers off that one cylinder and try the compression test again. You should have a sealed hole and the air would just get squeezed and released repeatedly. The compression tester would show something unless the piston was hosed. Burned valves and completely wasted head gaskets still typically push 15-30 PSI compression.

After all that, I have a suspesion that you have other issues as well. You mention that one rocker doesn't move as much as the others. That is a sign of a flat cam. It is looking like it is time to pull the engine and rebuild/replace. Everything you mention shows a worn out engine. I think I hear the cabinet maker making a coffin, and he is sizing it for your engine.
 

fordtrucks4ever

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DFW
SInce you removed the valve cover, just measure the spring height of exhaust valves. It should be very obvious if the valve is stuck open. IF The spring on #8 exhaust valve is taller than the others, you have a broken valve or wallered out valve seat.
 

Muddn7

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Dec 11, 2008
Messages
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Loc.
alamogordo, nm
backwoods check.... always sounds scary.....

I have found a quick way to replace a valvespring in the woods, and it might help you on your troubleshooting. Take an air tank, or compressor, hook it to a compression hose with the valve cover off. If you hear air from the valve, its either bent, or needs a head job. This ofcoarse is with both the valves closed and the vehicle not running. If your not sure, spray some wd40 on the seat and visualize. I wouldn't run the PSI over 80 !!!!!
 

bax

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Aug 22, 2005
Messages
14,491
It's going to be a burnt exhaust valve.
 
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