I have a 2007 Suburban with 328K miles on the 5.3L. Bought it almost new with 7K miles and have always run Mobil 1 Fully Synthetic. I've had absolutely no engine issues with it until about a week ago. I could feel a misfire on startup and while idling. Scanner showed cylinder #7 as the culprit. I ruled out everything possible on the outside of the engine so my next step was to take off the valve cover and check for a bad lifter. All rocker arms were perfect. While I was in there, I decided to replace the valve stems seals on that side. That went without any issues. Turned the engine over by hand a few times and all was well so I reassembled everything. Started her up and she was more noisy than I had expected but quickly quieted down. However, cylinder #7 was now misfiring all the time. Swapped the plug with #5 and she ran perfectly for several minutes. I then revved her to 3K RPMs and it immediately started to tick. Pulled the valve cover and there was about 1/8" or so of play in the rocker arm on #7. The pushrod sits about 3/16" lower than it should. I decided to take it for a short drive to see if V4 mode would free it up. To my surprise, when it entered V4 mode, the lifter tick went away completely which indicated to me that the AFM function of the lifter was working properly. The following day, I made a tool to try to jar the lifter internals free with no luck. I finally decided to pull the head. I was surprised to see that the AFM spring was properly extended and locked in place. I shot some air in the hole and it released properly. This did not appear to be your usual AFM lifter issue. Upon examination of the lifter, I could see that the socket was pressed in all the way and jammed in place. I tried pushing it out with compressed air with no luck. I removed the retainer and spring to try to remove the stuck parts. I used a pick with a bent tip and hammer to try to pull it out with no luck. I inserted a socket and tapped on it to try to jar it free with no luck. Tried more compressed air, nothing. Took my needle scaler to it but it wouldn't move. I gave it some light taps all around with a hammer then hit the end against the top of an anvil and it finally started moving. A few more taps and it was out. Below are photos of what I found. You can see that the disc appears to somehow have come out from between the socket and plunger and got jammed between the two causing a piece to break out locking them together. The pressure from the offset of the two being locked together is certainly what jammed them in the bore. Me pulling and banging on it must have separated the socket from the plunger allowing them to finally come out. I found no debris of any type inside the lifter. Everything is perfectly clean and looks like new. I see no scratches inside the body to indicate something caused the socket or plunger to lock up. Both the socket and plunger slide inside the body with no resistance. The plunger spring is in good working order. The only thing I found when disassembling the top of the engine was that 2 bolts at the left rear, #7 side of the VLOM valley cover were loose. There was no resistance when turning them with the socket. I don't see this being an issue as AFM was working properly even after the lifter locked up and the oil feed that hydraulically pressurizes the lifter internals is not the same as the one that feeds AFM. What are everyone's thoughts on this? I'm debating at to whether I should just drop in a new AFM lifter and call it good or if I should go full AFM delete. Also trying to decide if I should pull the pistons, clean and reinstall the oil rings and pop the pistons back in. The rocker arm bearings and trunnions seem to have more free play than I think they should have so I'm either going to buy replacement GM rockers or I may go with CHE trunnions since the cost would only be about $30 more than new rockers. The issue with the CHE trunnions is there's no real world info out there from people who have run them for more than 8K miles so I'm hesitant to be the guinea pig.