I just recently started having a missfire issue at 4500rpms and at 5500rpms. It was in and out at first, but then it began to happen more often. At first I just thought it was the cold weather since the car was having a tough time idling. Well I sought some advice and deicided to pull my plugs out and check them since I had another set that I could throw in. I was running the Denso IW24's which should be gapped at 0.028 inches. I get to cylinder #4 and it had filed down to 0.032 inches. That's a big difference when we are talking gapping here. So I throw in my spare set of NGK BPR7ES and so far no more problem. I did have about 30,000 miles on these plugs also. The plugs still looked good except I would have to reset the gap to make sure they are within specs, but I decided to use the new ones instead.
Just thought I would put this out there just in case someone else runs across a similar problem.

Barry

 

Hey.. found the thread.. I'm glad it worked out. It was weird, I had this turn up on my car with about 12k miles on it with the S-AFC.. For awhile I thought it was boost cut, then fuel cut, but it wasn't fuel cut because it wasn't harsh.. Figured maybe it was the cold weather, but as it got warmer it continued doing it.. I brought the car in for service at the time and told them I wanted my plugs changed, and they looked at me puzzled, said "Why would you want to change them, their good for over 15k miles" about a week later I ordered BPR7ES's also (one step colder than stock) and gapped them at .026 and went away completely..

So when you described the problem It sounded EXACTLY like what happened to me. Now every time I get a misfire or problem with stumbling, I pull the plugs, clean and re-gap them if their still good.. For the money, the BEST thing a person can do is change their plugs every 5000 miles (every oil change if you use the 5000 mile schedule like I do)

 

I just went through and checked the gapping on all the plugs that I pulled out and I got 0.032 on the #4, 0.028 on #3, 0.028 on #2, and 0.030 on #1. Like MJ said to me earlier, it was prolly heat that caused my gapping to go off. My car has always ran up on the EGT's. I never knew why at first. I was peaking at 940 C on only 18psi at redline in 4th. I got the mail in dynoflash then my EGT's would only peak at 900 C with the same boost. Lately I was hitting 1000 C with my TT custom tune with like 20~21psi at red in 4th and 5th. I didn't know why my EGT's were up so high when my AFR was in the 11.4 range. I finally figured out that my car can not run that much boost on pump gas when I ran some race fuel and my EGT's were only at 840 maxed out in 4th and 5th with the same boost. This heat is likely what caused the gap to be off. I only hope that I did not damage anything else with the extreme heat. The rule of thumb is not to ever exceed 880 C. My ECU had to have pulled timing at that boost level which will cause your EGT's to go higher. I do not believe that anything was damaged or at least I can not tell since my car runs a bit stronger than my buddies with similar mods.