My husband has a 91 ford f150 4.9L straight 6. It's idleing high like 2000rpm what would cause that?

He just replaced the valve cover gasket and changed the oil and filter. Its a manuel transmission and when he started it a back up it idled high. He tried replacing the idle air controller but that didn't help. Any suggestions

High idle speeds could also be due to a vacuum or air intake leak of unmetered air into the engine… Perhaps he loosened or broke a vacuum hose when changing the v/c gasket. Being a 91 truck, it could have some hardened hoses which can break or crack over time. Check all vacuum hoses and air intake ductwork for looseness, breaks, cracks, etc. Spray carb cleaner around vacuum lines/ ports or TB /intake manifold sealing areas while engine is idling… If idle speed alters, that indicates a vacuum leak. Could also be due to a defective TPS sensor if engine idles high or hunts around to various rpm's… Test for proper operation of the sensor.

http://www.justanswer.com/ford/80hq3-ford-f150-91-ford-f150-4-9l-l6-2wd-starts-stalls-consistently.html

TPS tests…
http://troubleshootmyvehicle.com/ford/4.9L-5.0L-5.8L/how-to-test-the-tps-1

It is vacuum leak. I don't think any car should idle that high even without an idle controller.
He probably forgot to plug the Positive Crankcase Ventilation (PCV) hose back in.
It is the hose that come from the intake manifold to the valve cover that he replaced the gasket.