I'm diagnosing a cooling problem I've had for a while. I keep losing coolant, and I can't find a leak. Obviously, my first thought based on that is that I have a blown head gasket, or possibly a cracked head and the coolant is getting into the combustion chamber. I have no evidence of coolant in my oil, or oil in my coolant. So today I pulled my plugs to see what they looked like. I took a pic. They look pretty much the same to me, though there is some slight variation (as expected). The plugs are all almost the same color, however, the two rear plugs on the driver's side are darker than the rest (bottom middle and right in picture). The middle one on the driver's side (bottom middle) is slightly glazed. The passenger side rear plug was a bit lighter than the rest (upper right).
All, in all though, I think they show no sign of any problems. However, I can't understand why the threads on all of them would be rusty.
What do you guys think?