i wouldnt be able to see it if it was bigger anyway my pos work internet is like half a step up from dial-up. but it looks like it was arcing to or over the insulator. precisely the reason i dont buy much delco stuff any more. only for my truck but that is sorta different

Misfire #6 came back today. I test drove last night after switching spark plug on #6. I'll check #3, which is the cofire cylinder, and switch that plug later today.

I'm ready to switch over the ICM and other coils from my 98 to troubleshoot while I wait for my new plug wires to show up.

Can anything non-electrical cause a misfire on the computer? Injector?

Low fuel pressure can give you random misfire. A bad injector can also create misfire.

The #3 plug shows a similar but smaller buildup of carbon on one side toward the sidepost. It is not glassy looking like on the #6 plug. But it does show peaks protruding toward the side post.

im not sure if you have checked yet, but i had the same problem a few years ago. take off your coils, then take off the icm under them. you will find lots and lots of corrosion where the icm contacts the mount. that is your ground. if you sand the corrosion off of the icm and its mount, you may solve your problem for free.

So I've replaced two spark plugs with iridium plugs gapped to .060 that I had taken out.

All coils and the ICM have been exchanged with my perfectly running 98 leSabre series II.

I have started and driven the car many times trying to get it to do this again. It finally did miss this morning after sitting for 40 minutes after a short drive to my coffee place. When I started it, immediately there was a noticeable miss, or unevenness. It was a single miss, not the loping heavy miss I had experienced at times before where I thought two cylinders were not firing right. Idled for 10 seconds hoping for engine light and it never came on. I was hoping to get a cylinder recorded.

I also think I feel a slight, short miss while driving that lasts a 1/2 second or so, but rare.

Injector? I have just put in Techron fuel system cleaner on this tank of gas, long after the initial problem.

Could coolant seeping into intake cause a plug to end up with the glassy deposit on #6 in the picture I posted?

I'm not losing coolant in any amount measurable on the reservoir. I do have a slight external seep on cooling at the bypass tube that'* been there since the mechanic did my Upper and lower gaskets with FelPro metal 3 summers ago. I plan to replace bypass tubes and put on a new tensioner along with the rear valve cover gasket, since that stuff has to come off.

I'm thinking:
switch injectors #6 and #4.
send oil sample to Blackstone to be sure no coolant is in coolant--last sample was late last fall and it was perfect.

What spark plugs are a correct replacement? AC Delco 41-101 are what'* in it and what'* recommended. They also were the original plugs based on part number on the old and current plugs.

I just pulled another plug in #5 to see what the front looked like and it shows that carbon buildup on the corner of the porcelain like #3 had and like #6 had a severe case of. The #5 could be scraped off with my fingernail but looked a little glassy rather than powdery.

i had a #6 misfire the first time i had to change my upper manifold. you put on the wires yet?

I didn't see this mentioned anywhere, what trim level is your Bonneville? It would also help if you could put this in your signature like most members have, this way as new people step into the conversation they can easily see what specific vehicle you have.

Iridium plugs are preferred for both the NA and supercharged Bonnevilles (if the supercharged engine is stock). What I ran into with the iridium plugs is after a couple mods they started glazing over and misfiring. After that I switched to autolite copper plugs. They need to be changed more frequently, but they withstand the boosted motor better.

I am putting on the wires and new iridium 41-101 plugs now. I put on premium AC Delco the part number without the 9 in front of it from Rockauto. I picked up the plugs today. I realized I should have started with plugs and wires, but I thought at 40,000 mi., neither could be the problem.

The difference I'm finding is the NAPA Belden wires from 2 or 3 years back state they are lower resistance suppression wires. This gives hotter spark. The long one for #6 measures at 500 ohm approx. The new AC Delco premium fitted wires have #6 at 3000 ohms.

I suspect the difference in resistance may change the spark character?

Upper intake was done 3 summers back by mechanic. On my 98, the leaking intake gave white plugs because the moisture blasted clean the cylinder and plug as it got sucked through the engine.