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Brad,
I found this website by googling "1990 Chevrolet Suburban surges off idle". I Also have a 1990 suburban that idles irradically and surges from a stop. I followed what you checked and did the same. I found another website that had a forum that the final problem ended up being the EGR Valve. I checked my EGR Valve and it was working fine. I removed it and there were hardly any deposits on it. At this point in my diagnosis I had checked EVERYTHING possible. Since this other site insisted there problem was the EGR, I called my local Napa and found the the valve is $120.00. Right now I'm a striking Union member of the Boeing Company, so I did not want to waste the money unless I knew this was the problem. I fabricated a EGR block off plate, installed it, and it made NO DIFFERENCE.
I found the problem by dumb luck!! I removed the IAC after performing the 2 resistance checks (checked out OK). It turned out that the tip of the air valve had a set of wear marks in it that were sticking in the throttle body casting. I push the valve to the "OPEN" position, cleaned it and reinstalled it. When I tightened it up, I turn it about an 1/4 turn past it's original location so the tip of the valve had a new area to seat against. I fired it up, problem GONE!!!! It drives great now.
I don't know if this will help you, but spent two days trying to figure out what was wrong.
Good Luck
Scott in Seattle
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