2015 Tahoe 5.3 with AFM. Had an AFM lifter go bad. Pulled the intake, heads and lifter baskets. Replaced all lifters with gm parts. Had heads resurfaced and leak checked. Replaced head gaskets, head bolts, intake manifold gaskets, spark plugs, wires and coils. Ran for about 15 miles and got P0300 misfire code. Hooked up scan tool and watched misfires and only 6 is misfiring. Removed spark plug on 6 and 4 to swap them. #6 was wet with oil. Did a compression test on #6 and #4 (4 for reference) and 6 is all over the place. Too low then too high. Pulled compression tester out and it was coated with oil. Removed exhaust manifold and #6 port is coated with oil. Removed the head and the #6 cylinder is full of oil. Any help or advice would be greatly appreciated. Thanks

 

you either have bad valve seals, or piston rings.

 

Thanks Mountain Man. That's what I thought. But this cylinder was clean when I removed the heads originally for the lifters. I am taking the head to the machine shop today to have them check the seals. The intake port is pretty clean with minor oil film, but the exhaust side is coated. The cylinder still has the original hone marks and looks just like all of the other cylinders. The thing that makes me think it isn't rings is that it filled the cylinder in less than 15 miles. I'd think that if it was coming from the rings it would take longer than that unless I had a major ring failure at which point I'd have severe cylinder scoring.

 

Sorry I should have specified ... I meant valve stem seals, not valve seats.