Having difficulty troubleshooting my truck. It is a 2008 GMC Sierra with the 5.3L flex fuel with AFM. Relatively low mileage (~70k).
It has had an issue with surging for a while, but has now gotten worse. It started as just a slightly noticeable "jerk" under steady cruising. Now it is a fairly violent surge while cruising and may drop itself out of cruise control. Speedometer will drop by 10MPH with no noticeable change in speed. It will have a high idle when coming to a stop, and then settle down after a few seconds. No codes or SES.
Troubleshooting done so far:Changed plugs & wires. No effect.
Attached ELM327 OBD scanner. Nothing out of the ordinary that I can tell. MAF reading fluctuates with surging. O2 sensors reading normally. Intake air temp reading seems to be off sometimes and choppy.
Changed MAF Sensor. No effect.
Fuel pressure is good, but fluctuates rapidly (~5psi) at idle and decel, and sometimes when surging.
Unplugged MAF circuit: One time, this seemed to eliminate the issue. 2nd time, it caused the ECU to enter overheat limp-home mode.
Checked/wiggled wiring at sensors and ECU. No effect
Sprayed starting fluid around intake to look for vacuum leak, no effect.
Found loose connection at battery, replaced terminal. No effect.
Took it to a dealer to have them diagnose. They diagnosed a bad torque converter. Swapped the TC, but problem persists.
Please let me know the next step for troubleshooting, will post results. I'd rather diagnose this properly than throw parts at it.
Thanks.