Rickmd

Background. Torque converter was replaced for the second time. Just got it back. Went for a test drive. Found manual shifting to be MUCH slower during normal driving. Dealership confirmed that the transmission had been reprogrammed as mandated by GM when the TC is replaced. They stated that the transmission needed time to relearn my driving style. OK. I’ll go with this.Decided to go for a drive to help the trans learn my driving style and test the premise that shift speed increases with more aggressive throttle. Found this still to be true even within the new transmission programing. However, a fifth gear shift above 6200 RPM resulted in a loud bang that was felt both thru my seat and feet. Very unsettling. My wife screamed “What was that?!” It felt like something broke.After collecting my wits, I manually shifted all gears and everything seemed to work. No vibrations or strange sounds. Next, I tested 4th-5th-6th gears increasing shift points by 500 RPM. Shifted well in all gears below 5500 RPM. However, shifts above 6000 RPM in 4-5- 6 gears did the same scary thing. I was concerned that something was going to come apart.I took it to the dealership this morning. More traffic (and police) made it difficult to duplicate the event. Finally found enough space to reproduce the event in 4th gear. The technician thinks that the gear pack of the next lower gear is not releasing early enough so that both gears are momentarily engaged at the same time. This cannot be good for my transmission (assuming it is the transmission?). They want me to drive the car for a while to see if the trans will learn and correct itself. I will admit that the severity of the event seems to be softening. Therefore, since the car performs ok below 6000 RPM, I’m willing to test their theory. I will put it to the test in the next 500 miles.What do you think? Is their theory reasonable? Has anyone else experienced anything like this?

kenownr

Are you sure your not hearing (feeling) the fuel shut-off between gears? It makes pretty good POP almost like hitting the rev limiter.
Next question for you -- where the hell did you find enough road to tag 6000 RPM's between 4th, 5th and sixth gears? I know Texas has some long straights but your talking 2 to 3 mile straights at speeds way above 150??

Rickmd

Thanks for the thought. I can't remember ever experiencing the fuel shut-off valve in the past. I have hit the rev limiter and the event I'm experiencing is much more physical. Regarding the gear shift points, you are correct. I miss-stated. I should have said the shift into 4th, 5th and 6th gear. Even so, your observation regarding required speed is correct. The Hwy 99 toll road around Houston has a posted speed of 75. On Sunday mornings, it is deserted. Starting in 5th at 75 MPH, it does take very long to reach 6000 RPM and then back out immediately. I couldn't do this on Monday because of traffic. We therefore did all further testing going from 4th to 5th. I agree, it is still too fast for the highway.

AORoads

"However, a fifth gear shift above 6200 RPM resulted in a loud bang ..." Isn't redline 6K rpm? If that's so, I'd say something was happening that may not have been appropriate to the operation of the engine.

WelderGuy

GM really shouldn't sell these cars to total flaming idiots.

falcon5619

NSC5

The horsepower peak of the LT1 is @ 6,000 RPM with a recommended maximum RPM of 6,600. Fuel shutoff RPM shutoff for this engine will vary by platform but will be between 6,400 and 6,600. And no it isn't going to magically explode when it hits 6,600 RPM.

Fuel will be interrupted at full throttle upshifts to allow faster shift completion but this should create mostly a sound without a major drivetrain jolt because that is what the fuel shutoff is designed to avoid. If you are getting these very rough shifts via manual shifting at moderate throttle then something is not behaving as intended and the tech could be correct that a clutch pack is slow to release so the transmission is momentarily trying to select two ranges simultaneously. Clutch to clutch architecture (which includes most current automatic transmissions) utilizes very precise shaft speed measurement and very closely timed electronic control of hydraulic actuators so that sequencing of releasing and applying clutch packs are exact. The transmission should be in a fast adapt mode and should quickly learn the timing BUT it is not a good idea to immediately try these extreme shifts until the controller adapts to the exact characteristics of your transmission which will take a couple of hundred miles of varied driving conditions. When they reflashed, your transmission controller parameters were set to the generic starting values for the 8L90 for your particular car configuration but each transmission and engine have some variance so the controller learns the best clutch release/apply timing and pressure based upon the results from shifts. Give it a little more time to adapt to less aggressive behavior (like it would have when the car was new and the controller was first adapting to the specific transmission/engine characteristics) and after that give it another test under aggressive driving.

Rickmd

Thank you NSC5 for such an excellent explanation. As you stated, red line for a ZO6 is 6600 RPM. I posted in the C7
General Discussion section hoping that if my problem was the result of GM's reprograming the transmission following the torque converter replacement, then appealing to a larger audience might uncover someone experiencing the same problem. Bad assumption.

I would not have known about the reprograming if I hadn't dug into why it was shifting so differently. You would think that you would be told to drive it like a new car until the trans relearns itself. My car has 32,000 miles on it and it never crossed my mined that it had to be broken in again since no work was done to it. Others having the TQ replaced be aware.

Regarding the technician, he said Thank You, that he enjoyed the ride.

AORoads

Thanks NSC5/!