I had the exact same problem last week. The same codes, the same great gas milage and the same cool running engine. You can search for my posts. I solved my problem by re-torquing my UIM/LIM bolts to the max 108 inlbs (previously 72 inlbs) and I also replaced all the small hoses on the PCV valve and put a clamp around the large one on the bottom for a better seal. I don't know which of these fixed the problem because they were done at the same time but it is a place for you to start looking. BTW when you fix the problem the engine runs warmer and the great milage goes away
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