Poor compression in one cylander at idle.

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RJG

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Stange situation. My daughters '63 Valiant has fairly consistant compression (125-130) on all but 1 cylinder, which is down at about 90. At idle the cylinder is missing, but it kicks in around 2000 rpm. Carbon build-up, sticky rings or worn cam were suspected.
A local mechanic (who knows the older cars) performed a leak test that held fine. So we assume no build-up problem.
Then he added oil directly into the cylinder and the compression jumped to almost 140. That should suggest no worn cam?
He did some standard cleaning treatments that did nothing. Plugs, wires, dist.cap have been changed and re-checked. The engine sounds great, with no knocking.
My daughter has been driving with an oil addative, but so far no change. So now we are scratching our heads.
The mechanic thought, maybe the compression ring is froze but the secondary ring is good. But he agrees that isn't a great explanation.
Thank you in advance for any thoughts or input you might have.
Rjg
 
Sounds like a broken ring or a sunken / burned exhaust valve to me. Since you have done a leakdown test with oil. That tells you that it is either rings or valves.
 
Mopar367 said:
Sounds like a broken ring or a sunken / burned exhaust valve to me. Since you have done a leakdown test with oil. That tells you that it is either rings or valves.
Click to expand...

x2 you will probably find a burnt valve, not totally smoked but only slightly burned
 
...............I bet the valve guides are very loose and the valve is wobbling around making the seat egg shaped.........kim...........
 
Had that problem with mine. #1 cylinder 6o psi, #7 cylinder 100 psi. Burnt exhaust valves in both of them. New set of Aero heads, problem gone along with the smoky start ups which of course was worn valve seals
 
My guess is a exhaust valve is getting weak.
I would run it until it is missing real bad, it may last along time.
Also check the lash on that cylinder, it may be too tight causing pressure to seep by.
 
One other test you can do is to pressurize bad cylinder with 100# of air when both valves are closed at BDC after backing off adjustment, and listen in intake and exhaust for air leaking. Then gently tap on top of rocker over both valves one at a time with hammer listening for escaping air and ability of valve to seal up. If head holds pressure, than you know that there is a ring problem.

As others have said you could have stuck compression rings from buildup of carbon, or broken ring. Sometimes you may get lucky with using ‘mechanic in a can’ snake oil treatment that promises to free stuck rings, improve compression, stop smoking, cure leaking body parts, and bring world peace, and sometimes not.
 
I have to go with these guys, since the oil raised the compression.
Oil won't raise it if it's a valve usually.
Only if it's rings.

I also have to agree with the mechanic, (ring/piston or cyclinder wall problems)


krazykuda said:
I'm going to go with rings....
Click to expand...

Oklacarcollecto said:
I agree. The compression jump with oil in the cylinder and the leak down test would make me think the valves are sealing
Click to expand...
 
trailbeast said:
i have to go with these guys, since the oil raised the compression.
Oil won't raise it if it's a valve usually.
Only if it's rings.

I also have to agree with the mechanic, (ring/piston or cyclinder wall problems)
Click to expand...

this x2
 
Thank you for all the input. Sounds like some engine work soon. Just in case, we will continue to run some additive, such as 'Mechanic in a Can' or Rislone in hopes of freeing up a tight ring or ?. Does any one have a high reccomendation FOR (or AGAINST) any one of the oil addatives that are available for such problems? I want to give it our best shot.
Thank you again,
Richard
 
Take a quart of dexron tranny fluid and put half in crankcase an other half down carb at high idle to keep it from stalling. If there is alot of gunk in crankcase it could be bad and clog up screen on pickup so be aware of this. Do the carb in open area cause it will blow alot of white smoke. We use to do this at Dodge dealer in the 70's on used cars to make them run a little better before they went on used car lot. I would say stuck or broken rings.
 
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