I'd suspect that your "backdoor hack" might have raised the line pressure a bit for firmer shifts at all times. But the TFs were already known for firm up and down shifts already, so were you trying to reap the benefits of a shift kit without having to install one?
It seems that the recent GM 4L60E family has a high-mileage issue where the pressure-determining solenoid fails and puts full line pressure into the unit. Usually after it has been in operation for an hour, from my observations. I had TWO Impalas that had this issue, once past about 150K miles.
I figured out what it was doing, so I didn't use a lot of throttle in normal driving. Once it was in OD on the highway, no problems. But on the 2005, it ended up cracking the flywheel, due to some stretch of the flywheel-to-torque converter bolts. Stretch that was not discovered until warranty had paid to replace TWO of them, plus having to buy a new GM Reman torque converter (which is a story in itself!). When it was in for another drivability issue, the tech got all bothered that the trans was going out. I told him to worry about the engine, I'd worry about the trans.
Problem was that to do the solenoid was about a 5 hour job, although the solenoids were not that expensive. I'd planned on getting that done, someday, but since they allegedly knew more about them than little 'ole me did, I opted for the GM Reman transaxle. 100K warranty, too. More of the car is under parts warranty now, for what it's worth. I've spent far too much money on a car with such a low resale value! But it's a durable car, otherwise, now with 225K on it. Starting to investigate an off-lease 300 for when I finally retire, or a lowered short wide single cab Ram Hemi?
Enjoy!
CBODY67
