RPMs increase & decrease at all speeds?

[Codfish]

Hi everyone,

Just bought an 09 Sierra with a 5.3, something seems off though- as I'm driving both slow and highway speeds the rpms go up and down around 250-300 rpms constantly while im keeping the pedal steady. I was thinking fuel pump but I tested it out today with cruise control and it drives perfectly steady with the cruise on. It feels like I'm pulling a trailer with the weight shifting back and forth. Please let me know if any of you have any insight as to what this might be.

 

Thank you!

[CamGTP]

Sounds like the torque converter locking and unlocking.

 

If you have a fancy scan tool you can watch the torque converter clutch on the scan tool to see what it's doing.

[Codfish]

I have a cheap reader unfortunately... if it was the torque converter though wouldn't you think it would continue doing it with cruise control engaged? I think that is my biggest hang up, its only doing it when cruise is off