Erratic idle, only when engine warmed up

Question

Not constant, smooth idle most of the time, occasional rpm drop of 100+, only when @ normal op temp., shudders then returns to smooth idle

Answer 1

That shutter is almost like a misfire condition, The CK ENG LT for a misfire condition does not have to set unless it is catalytic damaging, This year make and model , try a throttle body cleaning with a MAF sensor cleaning, Ford recommends the use of brake cleaner as the cleaning agent, and also look for any vacuum leaks especially at the PCV hose that hose tends to collapse under use,