I've got a 1999 4.6 that just turned over 200k miles. A week ago, I took a trip to Los Angeles. As part of my normal pre-drive checks, I noticed the coolant was a little low so I filled it back up to the cold level mark. In LA, I parked my truck on a down hill incline and the next day noticed there was a puddle of coolant right where the coolant reservoir overflow tube was. Didn't think much of it. While driving the truck home from LA, the engine started to run a little rough then the dreaded check engine light came on. When I got home, checked the codes and it was for a misfire on #6 cylinder. Since the plugs had ~90k miles on them, I replace them all. Test drove the truck and it was running great. Drove into work this morning and about 20 minutes/15 miles into my commute, the engine started running rough - like a cylinder was misfiring and the check engine light came back on. Got home this evening and checked the codes and found the same code for a misfire on #6 cylinder. I pulled the #6 plug wire and the spark plug end was wet with coolant. I blew out the spark plug hole and more coolant came out. I pulled the #5 and #7 plug wires and they were dry. I noticed the coolant was a little low, so I added a little more. I checked the oil and it looks good. Almost as golden as when I replaced it before my trip.

Question. Where is or how is the coolant leaking into the #6 spark plug cylinder hole? Cracked head? Partially blown head gasket? Leak in the intake gasket? Somewhere else?

Any help would be appreciated!

Dave