2008 GP surging, erratic throttle operation

2008 GP visited the dealer yesterday for TPMS re-set (replaced bad valve stem) and to snyc a new keyless transmitter (bad buttons on old one). Today I notice both part-throttle surging at 25-50mph and throttle operation that isn't sync'd with the amount of pedal I'm putting into it. Going up an on-ramp I kept pressing the pedal but the acceleration lagged behind pedal input. A little while later, going up a hill at 40mph it surged and lagged and shifted back and forth from 3rd to 4th. In traffic, a brief acceleration produced a throttle response that seemed like it went to 3/4 WOT when my foot was not even 1/4 into it. No message lights on the dash, no trouble messages or codes on the dash display. I recall reading a while back the electronic throttle has issues with the TPS or with the sensor and throttle motor not being in sync (software issue), but I can't find any info on the topic now. Any ideas what's going on?

Should have the vehicle scanned for codes....even thought the CE light is not on, some codes don't turn on the light unless it fails on two consecutive drive cycles...

Looking at the general information I have access to on the 2008 Grand Prix, I am not seeing anything that would help troubleshoot. If you decide to return to your dealership to get this looked into please let me know as I'm available to assist in the process however I can.

Regards,
Sarah
GM Customer Service

Thanks Tech II and Sarah. Today's it's working normally but I am overdue for the Maint II check-up and software update so I think I'll make an appt and see what transpires.

That thing with the TPMS valve stem corroding from the inside out on a car that's barely 4 years old is nuts. I'm a GM-lifer ('64 Tempest was my first car) and my father was a Mr. Goodwrench at the local Buick/Pontiac dealer for 30 years, but that's a really poor engineering job of the type I'd expect from one of the other Detroit makers. NHTSA complaint filed on that one. Hope they're recalled before I have to shell out $93 each for the other three...

I believe the government has a hand in the design and requirement of the new style tpms sensors. The government screws up everything they touch, so this would be no different. It is not just a Gm issue it is all vehicles that have those type of sensors. They corrode from the inside out and also to the valve cap, so when you try and twist them off the stem breaks and leaves you stranded.

Need to make sure any compressed air that's being used, has a good dryer for the Air-compressor.
You might even consider nitro for the tires.

Monitor + check maintenance schedule for anything that might of been forgotten.

Air filter / fuel filter
Clean MAF sensor, IAC, and throttle body.

Did these symptoms show up shortly after her physical?

Yes, the symptoms occurred on the way to and from work the day after the TPMS reset and keyless transmitted was sync'd. No problems at all since that day. The tip about nitrogen is one I heard from several good sources so I think I'll not be using the convenience store air pump any more.