Misfire caused by spark grounding
Greetings everybody, I was having problems with multiple misfires on my Jimmy and trying to find out the cause for quite a while and finally did today. It wasn't too many miles but it has been a couple of years that I changed the cap and rotor on this. For the last number of months it's progressively getting worse misfiring. Tried scoping the ignition system without much direction there. Took off the cap and rotor and noticed these spark traces signifying sparking to ground. Testing the continuity on the coil wire in the cap found that it had 12 ohms of resistance. Replaced the cap and rotor and now she runs beautifully again! Just wanted to note this and was wondering if anyone else had this problem. Thank you Happy New Year, Doug.
Nice job! I use Delco parts for secondary ignition for those reasons.
Doug Thanks for the images. What I wonder though, what is the resistance of the spark plug wires and the gap on the plugs. Was the spark taking the path of least resistance and traveling from the center coil terminal to the hold down screw because there was so much resistance in the wires and/or spark plug gap. Inquiring minds want to know!
I haven't checked the other resistances but I wanted to and will. There is something that caused the excessive resistance in the cap! And thinking more of it now more resistance is coming from downstream not necessarily from the coil path although that was causing the most misfires. Does that sound right?
Hi Doug, I think excessive gap on the plugs, or the wrong plugs would be a prime factor, because that would cause the coil to work harder. The gap between the rotor and cap also is a factor, I've also seen people actually file the metal tab end on rotors before instead of replacing the rotor. Needless to say, they had issues in a very short time
Hi Doug, A retired GM rep told me to advise you to stop turning the key to the right and the problem will not happen again. LOL I have seen much worse on a regular basis years ago. An old GM foreman I worked with told me to put a dab of die electric compound on the metal tang where it rides the center button of the distributor cap. This help keep condensation from collecting on the metal. The…
Yes I'm sure it did that explains when it was damper rainy it would be worse misfiring!
That's nothing compared what normally happens. Very common problem. I have two right now that the rotor tang is burnt off and is running around the outside of the coil terminal. On one, the carbon ball fell out of the cap and wedged under the tang. The whole pocket was solid black from all the spark scatter. Still ran but had misfires. Every once in awhile it'd kick back on the starter. That…
Great information Jesse thank you. I have pictures of the cap which I'll post of which it shows the spark excessively on one side of the post, it's on the right side showing retardation a bit I think. The distributor gear moves at least about 3/16 of an inch. So doing the hold down modification moving the distributor will probably help that.