1987 Chevy Suburban V10 4X4 VS Snap On MT2500

[Heckler45]

Good morning fellow Squarebody owners!! My 1987 Chevrolet Suburban 5.7 TBI 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. There's been NO MAJOR engine work on this vehicle from me AND ALL engine and behind the dash grounds are good. I double checked all the ground wires and just for kicks, installed another IAC, NO CHANGE. I tried a new MAP sensor, 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??? UPDATE: I changed out the ECM, no change. I even installed a new throttle body base gasket for kicks, no change. I disconnected the CTS, MAP, EGR, O2 sensor wire and TPS and the idle STILL creeps up ONLY when you plug in the 12 volt power lead, EVEN IF I USED the power lead that has a gator clip to the battery positive post. I can find NO ONE who remembers back if this was a problem on the first fuel injection models for Chevy trucks. Any help would be greatly appreciated!!

[Blue Bowtie]

Have you tried installing a temporary/secondary ground wire from the ECM to the engine/chassis?

[Heckler45]

Hey thanks Blue Bowtie! Yes, I have all new battery cables, battery and alternator too. I disconnected the alternator too AND run extra ground wires to the engine, frame, ECM and body. I just don't get it. The ONLY thing I haven't done is try the MT2500 on another 1987 Chevrolet Suburban. I did try my scanner on a friends 1994 Chevy 5.7 TBI and it performed flawless BUT....the '94 models will give you more data. I pulled the firewall wiring connector to clean the terminals AGAIN and nothing has changed. I even disconnected the 700R4 electrical connector and still no change. The CTS connector when open reads correctly on the scanner and when you jumper the two CTS wires, it also reads correctly. I wonder if the replacement Standard Ignition Parts brand of IAC has a winding issue that is not like the factory AC Delco units??? I know the older VW's will NOT charge the battery with a Standard Ignition Parts brand of voltage regulator......you MUST use a BOSCH brand of voltage regulator. AND some Honda Civics, Preludes and CVCC'S won't run more than a month with an ignition coil made in Singapore, gotta' stick with the factory Honda units. I can ONLY think maybe the early 1987 PROM won't send serial data correctly? Is there ANYBODY out there that has had this issue?