After going through a nightmare with a 4.6L, I discovered that the intake valve was "cupped" which made the valve stem too long. This would not allow the valve to close completely. A quick fix is to grind off about .50" of the valve stem. Either that, or remove the head and replace the valve. This valve became deformed due to a misfire that created a lot of heat. An intake valve should not be getting that hot since there is air flowing over it all the time. After the compression was restored, the cylinder is firing normally and the valve should not deform any longer. It should last. The only way to get this cylinder to seal, with air pressure in it, was to remove the rocker arm. After measuring other heads, to see how tall the valve stem was, I determined that there was around 50 to 60 thousands of an inch more valve stem on that intake valve. I ground it down, with magnets and rags surrounding it. Put the rocker arm back on, and had compression. Happy Dance!!!
I hope this helps all of you that are struggling with this dreaded issue. It appears to be a common problem which occurs after a misfire begins. This cylinder will then have no combustion, therefore no more exhaust leaving the cylinder. The air in the cylinder, will remain in the cylinder, and get extremely hot. The intake valve is fairly thin and cannot handle the heat. After a while, it will start cupping out and the valve stem will get longer and longer until the valve will no longer close.