So I've been having a really bad miss in my 99 with the 5.3L vortec. It's also accompanied by a cough/very mild backfire when I let off the throttle. Took all my spark plugs out and discovered they are all burnt in the same small spot. All 8 of them. Took them to autozone, and the guy said it looks it's running lean, and he's never seen anything like this on a spark plug. He then said his guess would be a pre or post ignition problem or timing issue. Any body got any experience with this issue? Picture of spark plug below. All 8 look identical.