rlchv70

Sorry if this is the wrong forum, please direct me to the right place if so.


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.
• 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.

Please let me know the next step for troubleshooting, will post results. I'd rather diagnose this properly than throw parts at it.


Thanks.

2xLS1

iTrader: (25)

I don't see how this could be happening.

Is AFM active when the problem is occurring? Misfires will cause cruise to disengage.

rlchv70

Occurs with and without AFM active.

2xLS1

iTrader: (25)

Try cleaning the throttle body. If it idles high after you do, you will have to do a throttle clean reset with a scanner.

rlchv70

So, took the truck to the dealer to let them diagnose it. They hooked it up to their Tech2 and said that the problem went away when they unlocked the torque converter. They insisted on replacing the entire transmission, so I took it home instead. I bought a new converter and installed it. Problem still persists.

Any thoughts guys? Anyone with access to a Tech2 in the chicagoland area?

Thanks!