Re: Valve issue
I'd do a compression/leak-down test to verify there's a problem with that cylinder first. If you find an issue, I'd pull the intake again and do the leak down test....or just apply pressurized air and keep it going, since I would imagine a bad valve or valve seat would just let the air flow. If it's pouring out that runner, you've verified your issue. Actually, I would think you could probably do this with the intake on as well, because I really can't see that changing the results any since the only runners it could come out of are those two.
Anyway, once you're sure that's your problem, and not just a bad injector, I'd take into account how long I'd had the issue and whether it had gotten worse over time before deciding if I should do it sooner rather than later. That's the backyard/broke mechanic in me talking. lol If I had the money, I'd be fixing it right away.....or buying a new set of heads and swapping them out. Doesn't take as long as one might think if you have the right tools.
If it is indeed leaking, you could potentially damage the head beyond repair. A new seat or valve is one thing, but it's possible it may be beyond that already. Other than taking the head off, the only way to really check is with some sort of bore scope. If you have to wait to do it, don't wait too long, and certainly don't run it hard/hot....and be prepared to find out you can't repair the head, just in case of a worst case scenario.
2004 Excursion 6.0
New HPOP/HPO system o-rings; Updated standpipes and dummy plugs
EGR delete; Rebuilt oil cooler; CCV collection mod
VR Black Diamond head gaskets and ARP studs
Oil and coolant bypass filters; Shell EC-1 coolant
Rebuilt turbo; 250 amp alt; 6000 watts of brain scrambling
One giant 240 pound English Mastiff riding shotgun!