Transmission, timing issues?

[GeeXPee]

Wasn't sure how to move my thread to the transmission forum so I just reposted...

Experiencing RPM surges and transmission slips while in Manual, tapshift mode in my 08' GXP. Driving in the mountains I try to engine brake if possible to avoid heat build up on the brakes, when I go from 3rd to 2nd at highway speeds the RPM will jump, come back down, and repeat, the car eventually slows down and it goes away. If I go from 2nd to 1st with my foot off the gas it's even worse as the car will drop out of gear completely, once I reengage the throttle it catches and slams into gear. If I put the car in Drive it functions just fine it seems, then again the downshift points don't really put the car into a lower gear at high rpm so it's hard to tell. Any idea what is going on here? Car is at the dealership now for a separate issue which they found lifter issues related to a ticking noise...they are repairing it now, this has been an ongoing issue though nothing new. This new RPM surging issue came after I made the appointment.

1. Could it be the lifters issue causing the RPM surging? Some issue with the PCV, vacuum?
2. Don't usually use Octane booster but I threw some in the tank for the hell of it last fill up, this issue started shortly after. Being at high altitude I just realized that the octane requirement is less. What's the chance the octane booster messed up my timing and is causing this issue?

 

[Coolride]

Wasn't sure how to move my thread to the transmission forum so I just reposted...

Experiencing RPM surges and transmission slips while in Manual, tapshift mode in my 08' GXP. Driving in the mountains I try to engine brake if possible to avoid heat build up on the brakes, when I go from 3rd to 2nd at highway speeds the RPM will jump, come back down, and repeat, the car eventually slows down and it goes away. If I go from 2nd to 1st with my foot off the gas it's even worse as the car will drop out of gear completely, once I reengage the throttle it catches and slams into gear. If I put the car in Drive it functions just fine it seems, then again the downshift points don't really put the car into a lower gear at high rpm so it's hard to tell. Any idea what is going on here? Car is at the dealership now for a separate issue which they found lifter issues related to a ticking noise...they are repairing it now, this has been an ongoing issue though nothing new. This new RPM surging issue came after I made the appointment.

1. Could it be the lifters issue causing the RPM surging? Some issue with the PCV, vacuum?
2. Don't usually use Octane booster but I threw some in the tank for the hell of it last fill up, this issue started shortly after. Being at high altitude I just realized that the octane requirement is less. What's the chance the octane booster messed up my timing and is causing this issue?

Vacuum along with tps would determine shifting points/load, and if a bad lifter wacks your vacuum on a downshift , it just might cause this erratic shift.

 

[GeeXPee]

Thanks good to see someone tried to help. The car has been in the shop for the last 2 weeks, they allegedly isolated it down to the 1-2, 2-3 band, GM TAC recommended replacement of the tranny vice trying to rebuild. I thought I would follow-up in case someone has the same issue in the future. They also replaced ALL of the lifters (ticking noise) and the ABS wheel hub (TC, ABS, SC throwing lights). Apparently my car is not exempt from any of the TSB anomalies. Over 10k in warranty repairs over the last 3 weeks. This has all happened while I've had the car up for sale, so I guess it's good that nobody has called...1 hit in the last 45 days, no one wants a V8 let alone a Pontiac V8, especially out west.