This is actually a fascinating diagnosis, in that it was the chemistry of the anti-seize compound which reacted and caused the backfire in the context of the aluminum engine block. I've personally encountered another issue which is the bane of alumnium blocks, and which is why the user in this case applied the anti-seize in the first place. Namely, it's very easy in aluminum blocks to cross-thread the spark-plug sockets, which will ultimately result in blowing out the cross-threaded plug (physically, as in shooting out of the engine block) because it's not being held in securely. Then you have to put a shield into the block to hold a new plug and it's a big mess.
So.. the spark was interrupted with antiseize because it was not conductive and you just don't grease up spark plugs. After all... they make special sockets for spark plugs and always use a small wratchet so you don't apply too much torque. If you don't have a small torque wrench.
The spark plug boot grease is more important than you would thing it would be. I understand the grease being a good way to not have the boot stick to the plug, but the containment of the 35000 volts seems like a much more important reason to use the grease
Carbon tracking down the side of a spark-plug is an indication that the plug itself was defective and had no center conductor or there was a conductive film on the outside of the plug (maybe Never Sieze??). There should be enough thread contact area that the 5-8KV firing voltage would break-down any minor contact resistance. Considering the number of spark plugs broken during removal, the use of an anti-sieze lubricant is the best insurance policy. Consider the number of copper and brass fittings applied using teflon tape, a much better insulator than metal filled paste such as Never Sieze, yet these applications exhibit excellent conductance.
I was surprised to read that the use of anti-sieze on spark plug threads caused this type of issue. The installer must have used an excessive amount of anti-sieze on the plugs. It is common practice in the automobile repair business to place a little anti sieze on the spark plug threads to prevent galvanic corrosion of the steel spark plug threads and aluminum head joint, which are commonplace in todays engines. I have been doing this well over 25 years and never had an issue. I do however use a very light coat of anti-sieze as a little goes a long way.I am glad to see that spark plug manufacturers have finally addressed the issue, but I am not sure all have, especially with plugs that designed long ago for older non-aluminum head engines, but carried over after the switch to aluminum heads.
"We found excessive amounts of anti-seize compound on the threads. This vehicle has a DIS ignition system, which uses the threads of the spark plugs as an electrical conductor."
Don't ALL spark plugs used in IC engines use the outer thread as the negative electrode? The high tension wire from the coil is one side of the circuit, the "load" is the gap between the electrodes, and the outer shell of the spark plug is the other side of the circuit.
Why is this "DIS" ignition system different?
Using NEVER-SEEZ or similar anti-sieze compound is not new. It was first employed decades ago when Chevrolet introduced the VEGA, which had an all aluminum engine, including the cylinder block. It was part of the procedure to apply a compound when changing spark plugs. Ask any "old-timer" who owned a VEGA, and serviced it himself. He'll tell you that IF you didn't apply the compound, after the recommended 10K or so miles, you COULD NOT remove the spark plugs. They became welded into the head.
And, there were kits available for those who stripped the threads. It consisted of a HELI-COIL insert, a special-sized drill bit, and the insertion tool. These kits were sold for more than just the VEGA. IF you went to an outboard motor shop, the mechanics there were very accustomed to performing that task also.
Properly torqued spark plugs don't "weld" to the cylinder head. They are dissimilar metals and thermal working can cause the plugs to tighten up. That and the fact that it can take as much as 150% of install torque to remove plugs they can gall.
The backfire problem can be made worse by the fact that many manufacturers are using distributorless ignitions, with dual lead coils. 4 cylinders use 2 coils with a floating secondary. The high-tension portion of the coil is not grounded. The circuit is made through BOTH spark plugs. The important part of this is that one plug fires normally, and the second plug fires BACKWARD from the sidewire to the center electrode. The system fires a wasted spark near TDC of the exhaust stroke. The center electrode can wear quickly and the required voltage to fire the two plug set can go way up. The addition of a anti-sieze that may increase the combined resistance could cause added problems. Remember this resistance is added twice as both plugs are active at every firing. The 1 & 4 plugs and the 2 & 3 plugs fire every time at TDC. DIY's should check plug pairs, to check for the wear on one of the two plugs if they have a misfire problem.
Well, maybe you're technically correct that two dissimilar metals do not "weld". However, I challenge you to tell that to the many customers that my brother saw in his garage 40 years ago who brought their VEGAs in for tuneups, and the spark plugs were "welded" into the cylinder head. That was long before all this fancy dual spark plug, no earth ground ignition system that you are talking about. The BIG thing back then was a "Transistorized" iginition system, so the points didn't bear the brunt of the ignition coil field collapse current. Plus, using a transistor allowed for more precise spark, since the make-break signal was a more perfect square wave shape.
I'm very familar witht the Vega. GM did all the learning curve with the Renolds 390 (If I recall correctly) The high silicon aluminum is touchy and was easier to gall to plugs than most AL alloys. Much maligned the funny part is that the Vega used an iron head with the aluminum block. An inverse of the standard set up.
You're absolutely correct. It was a REYNOLDS METALS high silicon content aluminum alloy. The idea was to eliminate the cast iron cylinder sleeve liner used in other aluminum-block engines. But, I also seem to remember that some VEGAs also were fitted w/ an aluminum head in the later years of their production cycle. Towards the end of its life there was a COSWORTH VEGA also. I remember that very well because a good friend of mine had one. When he got married, he sold it and bought the CHEVROLET MONZA, another "classic" vehicle from the skunkworks of GM technology!!!!
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