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One cylinder spark plug failure

Started by Tom Hoczyk , CLC 14044, May 10, 2016, 10:00:39 PM

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Tom Hoczyk , CLC 14044

This problem is by far one of the most intriguing I've come across on any car.  The car is a '62 Cadillac (Commercial Chassis), original 390 engine, 34,000 miles, resurrected this winter after a 21 year sleep.  When it runs well, it runs GREAT.  ONE cylinder is giving me fits.  Please read and absorb all the following.  I'd appreciate any help.

Point dwell and timing are correct.  Cap, rotor, spark plug wires, spark plugs have all been replaced with new.  With a new spark plug, the car idles and drives beautifully.  After about a week the car loses its smoothness and it's easy to tell that something is wrong.  The offending cylinder is always #6.  I have been using my induction timing light as a diagnostic tool.  When the car acts up, the timing light stops flashing on the #6 wire.  If I remove the cap end of the wire from the plug, spark in the timing light resumes normally.  If I get close to the top end of the spark plug, but do not actually attach it, the idle smooths out and the timing light resumes showing spark.  As soon as I actually attach the wire to the spark plug, the light in the timing light goes out and the engine idle loses its smoothness.  I have replaced this plug with new 3 times.  With each new plug, the problem goes away, but only for a while.  Why would spark stop when the wire is attached to the top of the #6 plug?  Many thanks in advance for any help.
Tom Hoczyk

wbdeford

Have you swapped wires with another cylinder with the same results still on #6?
1958 Sedan de Ville

Past:
1956 Fleetwood 75 Sedan
1957 Fleetwood 60 Special
1958 Miller-Meteor Futura Landau Duplex
1960 Coupe de Ville
1966 De Ville Convertible
1970 De Ville Convertible
1971 Eldorado Convertible
1979 Sedan de Ville
1980 Seville

Tom Hoczyk , CLC 14044

The old original spark wires were not in horrible condition and yes, I did put another (old) wire on #6.  Results were the same.  I do plan on swapping spark plugs between 2 cylinders, to see if the problem moves from cylinder to cylinder, but I haven't done that yet.
Tom Hoczyk

Bob Hoffmann CLC#96

Tom,
As far fetched as it sounds, it's looks like you're losing ground on that plug. Maybe run a jumper wire to a GOOD ground when it's mis-firing.
HTH, Bob
1968 Eldorado slick top ,white/red interior
2015 Holden Ute HSV Maloo red/black interior.
             
Too much fun is more than you can have.

Bobby B

Tom,
I'm going to throw my hat in and take a few stabs, since it's one of those " for the books" problems.  Are you replacing the plug with the same brand? If so, cross-reference it with another plug, and not a modern one, but a cheap, old-school copper core plug, and see what happens. I've been down this road before with a few cars, but it's most prevalent with vintage bikes. Triumphs and BSA's seem to only run best on a $1.99 Champion copper core plug. Same with the Harley Shovel head, but of a different brand. In no time flat, the plug will go bad and /or foul, even though it appears fine.  If you put a new one in, it will do the same thing within a very short time. It's a peculiar thing, but they seem to run best on a cheap old school plug of one specific brand. Fancy NGK, Split-Fires, Bosch, etc. make the bikes run horrible......
  Since it seems to be ONLY the #6 plug, and you say it's not fluffy, oily, or fouled, this seems to be directly electrical related, most likely to a misfire within/around the distributor cap. If your not getting cross-firing ( from the wires) within the firing order between adjacent cylinders, what else could it be? My guess is that the problem is related to your cap/rotor, maybe mis-alignment within the cap, hairline fracture in the cap near the #6 tower, etc. Take off the cap and hit the starter to see if the rotor seems to have a high/low spot, or some kind or defect when it comes up around the #6 tower on the cap. Run that baby in a dark garage and mist a little water around the cap, wires, plug boot ends, etc. and see what happens. I would check all of the above out first, then maybe try a new cap/ rotor from a different vendor.
                       Bobby
1947 Cadillac Series 62 Convertible Coupe
1968 Mustang Convertible
1973 Mustang Convertible
1969 Jaguar E-Type Roadster
1971 Datsun 240Z
1979 H-D FLH

Jeff Rosansky CLC #28373

Bobby has a good point.
You can rotate all the plug wires 1 spot on the cap. You will have to adjust the timing of course, but you can see if the problem follows the spot on the distributor or if it stays with the cylinder. May have a shot bushing that allows the rotor to miss that spot in the cap. Any play in the rotor/diatributor shaft?
Also wouldnt hurt to clean tbe plug threads on that cylinder. Be sure nothing falls into the cylinder. Does the plug torque down ok in thqt cylinder? Thinking maybe a lot of crap on tbe threads or a bad thread issue.
Does the plug wire come in contact with anything? Perhaps it is shorting out somewhere.

Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

cadman56

Hello Tom,
I'll throw my thoughts in here.  When you pull the wire loose from the spark plug what color is the fire that jumps?  I have found a fat orange or a thready blue fire is due to a bad condenser.  Are you running tin plated copper plug wires or resistance wires?  I ran tin plated copper wires on my 56's and never had any radio static issues & I found my engines always ran best & smoothest on NGK plugs.
One other thought you may reject but here goes:  You may have a valve trying to stick in that cylinder when the engine warms up.  Try to get a cold & hot compression test on it.
Good luck,  Larry
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville (sold)
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible (Sold)
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

Jon S

#7
I think we are missing the key part of Tom's post - "If I get close to the top end of the spark plug, but do not actually attach it, the idle smooths out and the timing light resumes showing spark.  As soon as I actually attach the wire to the spark plug, the light in the timing light goes out and the engine idle loses its smoothness."

It almost sounds like excessive resistance, grounding or something else strange going on.  Don't think it's in the Distributor. 

Changing the spark plug resolves the problem temporarily which leads me to think spark plug; not distributor. The question is why does the problem replicate itself.

I agree with using a copper spark plug like a J12Y
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT