1987 Chevrolet Suburban 4X4 5.7 TBI

[Burb restorer]

Good morning fellow Squarebody owners!! My 1987 Chevrolet Suburban started giving low idle stall out issues and I hooked up my Snap On MT2500 scanner and found the issue. A bad IAC valve. No problem. I changed it and it runs great........EXCEPT.........when I plug in the cigarette lighter positive lead for the MT2500 ALDL plug power cord, the idle speed will VERY SLOWLY slowly creep upward. After all of those years on the workroom floor as an automotive technician, the ONLY thing I remember with the early scanners were the Tech 1's would do this ONLY if someone left one of the original ground wires off the engine or cylinder head during any kind of major engine work. NO MAJOR engine work on this vehicle from me. I double checked all the ground wires and just for kicks, installed another IAC, NO CHANGE. I know parts today are NOTHING like they used to be so I have eliminated the IAC for sure. I am back on it today and now I am really starting to scratch my head. If I have the scanner HOOKED UP and drive it, the idle speed, when stopped at a traffic light, will SLOWLY creep up and I have to really hold the brake. The idle will get to about 1250 rpm in about 60 seconds. I can hear the idle with the scanner DISCONNECTED and I know it's close to where it's supposed to be BUT NOT with the scanner hooked up. Vacuum leaks? Nope, I even hooked up a vacuum gauge (NO laughing at this old tried and true method) and it's holding at 22 inches of vacuum, steady and absolutely NO bouncing of the gauge needle at idle. The factory Chevy repair manual points to the IAC OR the ECM at the very end of the page on TBI 5.7 idle issues. I know from years of being a technician, the ECM is the last thing I would consider changing................has ANYBODY ever had THIS happen???