It is a 4.0 1994. I am going to have the codes checked this weekend. I cleaned the idle motor. Nothing. Yesterday I drove to work, no check engine light on, ran good. Then on the way home the check engine light was on, and the RPM gauge read 0. But the truck ran fine. I changed the engine coolant sensor on Monday. Still the same. No vaccum leaks that I can hear. | 
