Some GM vehicles use the MAP sensor as a baro sensor as the key is turned on but but before startup to read the current barometric pressure.
The '94 turbodiesel pickup I'm working with now has ONE sensor... baro BEFORE startup, then EGR apply vacuum AFTER startup. I know some gassers are set up that way, as well.
As a cheap diagnostic test you might try "T"ing a vacuum gauge into the EGR apply line and see if it has vacuum when the EGR would be applied, typically at crusing speeds.
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Today's Featured Article - Talk of the Town: The Saga of Grandpa's Tractor - by The following saga is from the Tractor Talk Discussion Forum. Someone. The saga starts with the following message: Hey guys I have a decision to make. I know what you all will probably suggest and it will probably agree with me way down inside, but here it is. I have a picture blown up and framed in my "tractor room" of a Farmall M. It was my Grandpa's tractor, of which whom I never got to meet. He froze to death getting this tractor out of the barn to pull a truck out of the ditch before I was born. Anyway my dad and aunt had to sell it at the auction,
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