2001 Dodge Ram reving engine

This truck is reving at about 2200rpm when in park. The cel on this truck is on almost all the time and has been since practically new. We have had it in to Dodge while still under warranty and they never gave us an answer. The code that comes up indicates an evap. Problem when these truck were new dodge told us our furl caps weren't on tight enough. We made sure to tighten them three clicks as advised. The problem persited and still does. We've simply learned to live with it. However now the engine is reving and we can't get any code other than evap. Problem. Clearing this code gives no results. Throttle linkage is all free and operating normally. With no codes I'm not sure were to start. I'm thinking throttle positioning sensor? Any help is appreciated.