2008 5.3L engine surging

rlchv70 · December 3, 2016

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.

The Zip · December 10, 2016

I remember similar surging on a vehicle, change out your alternator and see what happens

rlchv70 · December 11, 2016

I remember similar surging on a vehicle, change out your alternator and see what happens

How would that affect anything? Alternator seems to be putting out good voltage.

The Zip · December 11, 2016

I had a vehicle with similar surging and my alternator was also appearing to put out good voltage. I disassembled the alternator to find I had worn magnets. I replaced the magnets and no more surging, changing out the alternator vs the magnets was just an easier recommendation. The alternator may not be the ticket, I only provided the troubleshooting suggestion because of a similar occurrence. But whatever works for you.

rlchv70 · February 16, 2017

Still looking for suggestions here. Some more things that I've noticed and things that I've tried.

Voltage at 15V. Disconnected alternator harness, but not charging wire. Voltage came down to less than 14V. No change in behavior.

Rough, unstable idle.

Hesitates sometimes on moderate acceleration.

Always runs well at WOT. This seems to indicate to me that it is a problem operating in closed loop.

Hooked up a generic scan tool and found the following:

TPS fluctuates when surging. There is a large difference (~10%) between commanded, actual, and relative TPS when surging.

Outside air temperature reads low (17 vs 42)

Intake air temp is erratic at low temps. Bypassed the IAT with a resistor with no change in symptoms.

Is the fuel pressure fluctuation normal? Not sure how the returnless systems should act. Don't want to change this out to find that it is working normal.

Thank you.