Changed the plugs on my 4.6 today (109,000 miles). The cop boots on cylinders 3, 4 and 6 looked like they had rust on them when I pulled them out. When I went to remove the plugs in those cylinders the plugs were only finger tight...so I'm assuming combustion gasses leaking past the loose plug caused them to look that way. Has anyone seen this before? I've owned the truck since it was new and this is the first time the plugs have been out to my knowledge.