My 89 with 130K needed a new Egr valve, it was not functioning and my mileage was terrible. The truck vibrated badly at idle, and ran pretty weak. I replaced it and viola, smooth idle. My Egr had carbon build up, I think from a leaky PCV valve grommet and stuck PCV valve, but I am guessing. Could that make sense? The timing was also pretty far advanced out of tune. I fixed things one at a time, and checked the idle, and I think the EGR was the culprit for idle vibration and power at high speed, but all combined really helped. I was told by the previos owner through a 2nd party that a tranny shop said it was shifting funny because of the EGR as well, but I didnt have it long enought before I replaced the EGR to notice.