I just bought an ex pg and e 1989 super duty. 6.9 diesel. There is a bad vibration/shake that comes and goes at idle. The truck runs great on the hiway. The vibration goes away when you rev the motor. At first I thought this was a loose mount but I dont think so now. ? fuel injection. Any ideas?

 

That almost sounds like a nozzle (injector) is taking a crap. The higher rpm's when running at operating speeds will force the bad nozzle to work and then smooth out, but this will soon pass as the nozzle worsens. You can check them out by breaking the nut on the top of the nozzle while the engine is at idle, but give it some to refuel between nozzles. This will be the same as shorting out a spark plug. Possibility of fuel system getting air in it too. If you have one of those water separators on the firewall, raise the drain valve and let it slam shut several times.
Let us know how you make out.

BE CAREFUL AROUND MOVING PARTS !!!!!!!!!!!!!!!!!