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Idle is fine when first starting cold. That is about 1000-1100 PRMs. As idle starts to go down and finally reaches 850 the idle is rough. Other than that, truck runs great, smooth with no misses. TPS MAP and IAC all check good. I have no codes in either test. 11 all the way. I just put new Motorcraft copper plugs in and gaped at 44K. (Glad I checked them for proper gap). I find no unmetered air and used propane to check. The idle is annoying and I know it's not right so it bugs me. What do you folks think I should do to find this problem.
My vote is a vacuum leak. Those things are sneaky and can hide in broken diaphragms, bad pcv valves, leaking brake boosters, stuck canister solenoids, etc.
