The DFM lifters on cylinders #6 and #8 went bad on our L84 5.3 V8 at 585 miles. The flashing check engine light went off every time the lifters would stick in the collapsed position, four events in total and four corresponding visits to the dealership. The truck spent 51 days combined at the dealer and was driven several hundred miles by the dealership to recreate the issue until a GM engineer was called in and it was repaired. This was prior to the current TSB guidance where they should have replaced all 16 lifters; only four were replaced.

The issue has not yet returned as of approximately 11,800 miles, but with 12 original lifters still present, it will inevitably happen again. I now drive the truck exclusively in L7 (8-speed transmission) to disable DFM and the drivability is way better without that awful system active and I only lose 1 MPG without DFM and 8th gear. Given that out of warranty this repair is $5,000+, we will be selling the truck before the warranty period has passed; it is too much of a liability and I regret purchasing it.