This has been an on going problem for me. It happened with the 3F-E, and now it's happening with the 2F-E. The head was rebuilt and checked for cracks, I used an OEM head gasket, and it runs great till that plug is clogged up. And it's only the #5 plug. I use NGK plugs. I have a theory about the little coolant lines that run in the 3F-E intake. I think that there may be a crack there and the coolant is slowly getting burned off. It may be a head gasket, but I really doubt it. The oil has no water in it, and the coolant has no oil in it. Should I take the coolant lines from the intake, and spray some carb cleaner down there to see what happens? Thoughts? I am really lost on this.