1990 F150 5.8 / 351 Very high idle. On cold start the engine moves from normal high idle to low idle. As the engine starts to warm-up fast idle starts to kick back in until the engine is fully hot and is running some where between 3k and 4k RPMs. I can unplug the air bypass valve and the engine will return to a poor idle. I found that you can also remove the manifold vacuum line to the MAP sensor and putting your finger over the manifold vacuum port, the engine will return to an idle. Off hand, I do not detect a vacuum leak. I have replaced the Air bypass valve, the air temperature sensor and the map sensor with no luck. Has anyone a suggestions?

 

i think it still sounds like a vacuum leak. i had a car that did the same and it was a faulty intake gasket. hard to hear because of the high rpm's that the computer makes the engine spin to keep itself running. try spraying eather around suspected gasket areas with it running. if you hear an increase in rpm, you have found your leak