Uhmm, are the ones cracking on the firewall side or in the front. Are the sparkplugs cracking inside the porcelin or on the spark plugs where the wires connect. How hot is the engine getting. Try and run a different type of wire set. Taylor makes a nice set or MSD'*. I have Nology but for the price of these, it really isnt worth the money.

It depends on what you call high altitude. I usually start these trips at around 1k feet above sea level and the hills run up into the 4500k range. I used to live in areas 3 - 4k feet above sea level with no issues. I did not have a scanner hooked up at the times that this happened.

2 cracked on the firewall side (#2 & #4) and one on the front (#3). The porcelain insulator right below where the wire connects is developing visible cracks and misfiring. The engine at its hottest has gotten into the 230 range, although that is very rare. At this point, I blame the plugs, but if the new NGKs start cracking, I will be going to a different set of wires.

It seems like the wires are putting some stress on the sparkplugs if they are cracking below where the wire connects. Are you crimping the wires to the plugs, if so try not too and just click them on. What type of wires are you using and see if any are tight because maybe they need some slack so the wires are not pulling on the spark plugs. 230 your temps should never climb that high if so see whats causing your temps to reach those levels even in traffic.

All the wires have slack and I just snap them on however tight they are from the manufacturer. I don't add any crimp to them. The wires are the Duralast brand from Autozone. I have not ruled out the wires simply because Autozone parts have never worked particularly well for me. I agree that the temps are a little high and I have a few thoughts on why this might be. I am leaking coolant out of the engine (LIM gaskets, most likely) and I suspect there may be a blockage in the radiator. I plan to backflush the radiator when I tear things down for the UIM/LIM gasket repair. Regardless, it is not often that it gets that hot and even when it does, the plugs should be able to withstand that heat.

I have a 2000 Bonneville, engine code K. Similar problem. I had a champion copper top spark plug installed. I put in new plugs about a year ago and noticed a misfire on cyclinder #4 (firewall side of engine). Crack was in the upper part of the plug. I replaced all the plugs with the same type. Car drove fine for about 2 months. Now recently it started misfiring again, cyclinder #4 under load with TCC engaged. Removed cyclinder #4 plug and it shows some minor cracks in it which I assume are causing the problem. Did the problem in this thread ever get resolved?

I haven't had another crack since switching to NGK, so in that respect, yes, this has been resolved.