I am replacing valve cover seals, installing nwp spacers, and painting my manifold. I realized the other day that on the insides of the manifolds they are all really dirty (175k miles 99 cali spec)... except the far left cylinder if you're looking at the engine from the front of the car looks like it's barely been used. Now I've only had the car for 10k miles, and 7k miles ago i got a misfire from one of the coils, but i had a spare car i wrecked with fairly new coils on it so i swapped them over and it's been working fine ever since

 

Most likely a bad injector.

Less likely a head gasket issue.