2006 GMC Denali shudders at 40-59 MPH but stops when given more gas.
The next time you feel the shudder, very lightly step on the brake pedal while maintaining speed.
If the shudder stops, the problem is the lock-up torque converter clutch is shuddering.
Add this to the transmission fluid to stop the clutch shudder.
Most parts stores sell this.
Misfires often are more detectable in that range of speed also, because the torque converter is usually locking, the rpm’s are low, and the engine is almost lugging (especially in the 40-50 mph range).
How long since the plugs and plug wires were changed? I think GM recommends plugs every 100k miles. Is the check engine light coming on at all?
Tester, is there a Lubegard product you’d recommend for slipping / delayed engagement of gears (not necessarily overdrive)? My Buick slips, or rather shudders, between 1st and 2nd at times. Then it will hard shift (per the tcm’s command). Most likely it’s a mechanical issue at this point, but I plan to change the fluid again and would put in an additive if it is worth doing. I’m not going to do a rebuild. When the trans dies, the car goes with it.
I’d add Seafoam Trans Tune to the fluid prior to having the fluid flushed.
And if it doesn’t help, there’s something mechanically wrong inside the transmission.