mattc01

I have a 97 f-150 4.2 I had a severe misfire on cylinder 6. checked compression and compression. all checked ok. I also had spark. fuel injectors spraying ok. long story short pulled the upper intake and found that all the ports were black and #6 was cleaning instantly saying coolant. I replaced the lower intake gasket but not the upper as it looked perfect. It was hard plastic gasket. No tears or anything. Changed the oil put new coolant in. Cleared all the codes and testdrove truck. I went around the block and down the street and the truck drove perfect, perfect idle and everything. Went a little further testdrive and found that the truck was no longer chugging but however had a little bit of a hesistation under load. idle is perfect. Checked the codes while driving and came up with p0141 bank 1 sensor 2 oxygen sensor. This code was not here before. Any ideas? I do have a exhaust leak but its back by the right rear wheel in the muffler. I doubt thats causing it.