As I continue to refine my car that is now at the body shop prep'd and awaiting a paint refresh on Monday the shop owner told me it seemed like the car had a miss while idling. Since I had just replaced the old plug wires with new Magnecor 8mm wires last week I immediately thought that maybe one of the wires wasn't fully seated. So to be absolutely sure I spent the morning taking the A/C unit out and then working from front to rear to pull each plug, clean it and then re-install it paying special attention to ensuring the wire "clicks" on to the plug and is fully seated in the distro cap.

Now for the interesting or weird part. When I took the 3A plug out I saw that the ground electrode was touching the center electrode. Since I was the one who put the plugs in and all the others were gapped correctly at .024 I was left to wonder if I somehow failed to set the gap on that one or some other event inside the cylinder was going on?

I re-installed the plug now with the correct gap and finished the rest of the plugs and wires check. I then started the car and voila' No miss, no tic tic tic, no hard starting. So either I am an idiot (most likely) or something weird is going on at the top of cylinder 3A. A plug with no gap would definitely affect the engine idle and operation since it essentially would be operating on only 11 of 12 cylinders. I didn't run the engine for long since the car is all prep'd for painting with the engine compartment taped off to shield from overspray.

 

Did you check the plugs before you installed them.

I changed the plugs in mine and the plugs I bought from my local shop ( the correct number) had a longer threaded section so it could have touched the piston. I took the plugs back with one of the originals and they ordered what I needed.

 

I have AC/Delco plugs installed. I am the one who put them in (and gapped them).

Once the car comes out of painting I will re-check the 3A plug to see if it is still gapped or if it is being touched by the piston. It never occurred to me that the plug length would not be correct if I was getting the plug number indicated in the catalog. Hmmm....

Good news was that plug tip health appeared to be good on all of them. No oily buildup or electrode discoloration noted.

 

Maybe a rogue lump of carbon? Or even, a rogue lump of ceramic or grit from the plug or plug well when you removed the old ones?

 

I agree with Greg.

The HE spark plug is a looooong way from the piston crown.

I run NGK BP?????? spark plugs, and that "P" means a Projected Nose style, which has the tip projected some 10+mm from the plug snout. The Original NGK/Jaguar plugs DO NOT have the "P" specified, BUT, that puts the spark somewhat up inside the plug casing, and fouling is easier than with the "P" plugs.

Just my findings, and usage over 20 odd years with the V12.

When you take the cylinder head off, look at the position of the spark plug tip, and you will understand a tad more what I mean.

Starting any V12 just to see if it runs, is a no no. If you start one, run it until Normal Temp is reached, then shut it down. Clearly, if the thing is running crap, clanging and banging, shut it down.

 

Life is just so much more complicated. That includes what was once a simple task. Spark plugs. Can't rely on the accuracy of a suppliers catalogue.


I recently swapped in six new HGK spark plugs in my daughter's V6 powered VW Passat.
I noted the ones I selected from an on line vendor were a "tad" different from the ones I removed. In the car were plugs, also NGK with multiple ground posts.
The new ones with more conventional single ground post.


So,. I looked carefully to see that the in the chamber length was the same. Yes. So far, OK. Then got out a proper guage for the gap. All but one just fine at .032. Set it.
In they went. A dab of antiseize on each thread.


It fired up immediately on cranking. Ran sweet....


Carl

 

She doesn't sound bad at all. There was a hint of a miss before and the idle would surge about 25 rpm or so I am guessing the computer kept trying to find the right spot to deal with the 3A cylinder not firing correctly.

The new plug wires, double checking the connections of all the boots and checking the plugs and gap has everything in the best shape I can think of.

Only thing left to do I can think of is the glass of water test sitting on the manifold with the motor running to see if she is running as designed or if three is still a miss somewhere.

 

HA, Orange Blossom balanced a British penny on its edge on his, and it stayed put.

Good luck.