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I have a 2003 f150 FX4 with the 5.4 triton and when idling in park the truck revs up every so often and won’t hold a steady idle but in drive it idles perfect only maybe just a hair low about 550-600
If you are running the defrost, that spins the AC compressor and the idle will fluctuate as the compressor kicks on and off
Usually, it's not very noticeable
How much is yours fluctuating?
I'd check the fuel trims with a scan tool and check the intake manifold for a vacuum leak at the plenum
Any unmetered air getting to the engine will make the processor send fuel, that will make the idle fluctuate
Any codes?