89 F 150 VERY high idle, unplug vacuum to MAP sensor, all's well

[rosenbarn]

I have a 89 f150 that idles very high (over 2000 rpm) as soon as it is started and doesn't seem to change after it warms up. Have changed the throttle position sensor, checked egr, changed map sensor, checked idle air control. When I unplug the vacum hose going to the map sensor it idles smooth and normal. Is it possible that the upper intake manifold gasket is leaking so bad that the map sensor is compensating for the loss in vacuum by sending more fuel? (Can hear a hissing but can't tell where ). If anybody has any other ideas to try or suggestions, it would be greatly appreciated. I'm a DIY person, and would rather see if I can fix it myself before taking it to the mechanics and paying their fee.