will be interesting to see if it unsticks. feels like it's worth a try
from what I understand build up crude is what happens most of the time.. when you go into 4cyl mod, internally it sends oil pressure to a extra port in these special lifters, that oil pressure releases 2 pins and the lifter collapses, the push rod doesn't open the valve on those cyl. when it goes back to v8 mode, the oil pressure is vented and the next time the lifter is on the closed side of the cam, a spring pops the internal part of the lifter up and the pins lock it into full size again.
over time the internal parts can get built up gunk, or a varnish like substance. the internal part sticks down and you get the tap tap tap that slowly kills everything from banging around and spreading metal thru the rest of the engine. usually hurting main and rod bearings too.
if you can clean up that enough for it to release, it may but really even be cleaning it, just getting hot enough to expand and the banging releases it. you could be good for a while, best to make sure it never goes into 4cyl mode again thou.
other ways they stick is lack of correct lube, either to long between oil changes, crap oil, low oil pressure to the lifters, could be from bypassing somewhere else, clogged vlom screen under the oil pressure sensor, basically anything that happens that keeps it from getting lubed, doesn't have to be low pressure that would show up on your dash gauge. the internal parts of the lifter can get galled up, and get stuck so you'd have to hammer than apart.
if it is a lifter, and not a broken valve spring, something I feel would be a good idea to check before you're Tennessee tune-up, broken valve springs do happen and tap tap till they drop the valve into the piston and lunch the whole motor.
but if that doesn't work there's a guy that sells a tool, basically just a metal rod with a ground at a weird angle. you pull the intake, stick it down thru a oil passageway to the stuck lifter. smack it with a hammer, the hope is to shock the lifter enough to manually release. in one of his videos one is so stuck he hits with short air hammer blasts, but it does release. once released you have to have the ecm tuned to never. engage afm 4 cyl mode again or it will stick again at the first afm activation. there's also a feeling that this is a temp fix or just a horrible hack job. since you still have all the damaged parts still in there. afm lifters even when turned off in the ecm when new still seem to fail before normal lifters most of the time. that said these days parts quality is very lacking everywhere. you could possibly do a full delete with all the most expensive parts and it still fails.
it all depends on your level of skills. if need be I could probably pull the heads and swap the cam in a ls in a day. I just have zero desire to do that, feels like 200 more important projects I can't find time to do. it would be my very last resort. I would pull the valve cover, inspect the spring, stick a bore scope in the spark plug hole to see if a seat dropped, then once confirmed I'd get the tool and beat the lifter with a hammer haha. at least I'd enjoy that even if it didn't work. but I fully understand that is you're not comfortable pulling the intake and stuff, or don't have a place to work on it. one guy on here had to do the hammer release trick in a parking garage of his apartment, in winter up north. sounded rough, but he got it.
the Tennessee tune up is definitely a thing to try before spending 3k-7k at a shop. if I had to put $7k into my truck, I think I'd roll it into the street put a free take sign on it and happily never see it again, spend that 7k on just about anything else, even a older Gen suburban or XL and never look back haha
on a side note, when I first got my truck, I knew about the afm nonsense, so I brought hp tuners with me(I already had it from tuning my other cars) to turn it off in the ecm before I even got on the interstate heading home. 1k mile trip, once home I pulled the intake off, pulled the vlom off, installed the oil pressure block off in the afm supply passageway, clipped the gasket on the other side of the ports where the oil pressure goes to release the lifter. that way there's no chance at all any oil pressure building up in the release circuit of the afm lifters.
I feel like it's the best mod you can do to try and avoid afm issues short of a full delete. which I may still have to do at some point. but spending a few hours and 20$ to give it the best chance of survival seemed like a good idea at the time.
now turning it off in the ecm, hpt tuners is kinda expensive for a one time use and not needed unless you wanna diy tune. you can send off the ecm to get it disabled for under 100$ if you have a few days down time. or most recommend getting a black bear tune, they will tune the engine and tranny, also will disable afm for you. the afm disable at that point for them is just a drop down box and save.
good luck with it.
