First off, does your "service engine soon" light illuminate briefly when you start the truck? Make sure the bulb works...
Second, put the ECM into test mode by shorting pins "A" and "B" on the diagnostic connector to the left of the steering column under the dash. I believe the two pins are: "A", upper right corner of the connector and "B", directly to the left of "A" or directly below, I can't remember. Someone help me out here...
Turn the ignition on. The "service" light should blink a code 12 (flash, pause, flash flash) to confirm operation, then any stored codes which can be deciphered with a Chilton's, Hayne's or factory manual. Each code, including code 12, will flash three consecutive times.
That would be my first suggestion. If you have a vacuum leak, it may pop a code for the MAP sensor. Then you can use the old "garden hose in the ear" trick to isolate it. Or you may get something else from the ECM so you can go in that direction.
Hope this helps!
Haze gray and underway...<font color=blue>or blue, in my case.</font color=blue>
<a target="_blank" href=http://community.webshots.com/album/26246038MbQsYWKcbo>U.S.S. Nimitz II</a>
