2001 Ford f 150 4.2

Have you checked the small rubber vacuum 90 degree fittings under upper intake, one in center toward front and one toward back of upper intake passenger side both are on the under side. The rubber fitting boots that were OEM at time of build were faulty and break easy but the new replacements are better and last and work well just make sure you ask for the new style at the dealership service desk as it the only place to get them parts stores do not have them. This may be it or not but mine did it twice and both times it was the rubber vac 90 degree boots under the intake and this last time I bought they were much better made.

Also the PCV is easy to pop out of place make sure it is seated well, its on passenger side valve cover near firewall and its vacuum hose runs up under the upper intake so its hard to get your fingers in there but is possible, may need a mirror and a light to see those connections.