This bulletin applies to 2018 GMC Yukon and Yukon XL vehicles equipped with a 5.3L or 6.2L engine. A vehicle may have a concern of a misfire P0300 on Bank 1 that may or may not be felt while driving.
The timing may retard up to -16 degrees while the concern is duplicated. There may be a noticeable tapping noise from the engine at idle that could be heard while standing next to the vehicle.
This concern may be picked up as a knock activity on the affected cylinder(s) which is causing the ECM to retard timing on those cylinders. This can cause weak firing pulses on Bank 1 cylinders which contributes to counting misfires.
If this condition is found, inspect the HPFP lifter roller bearing for failure (making noises and/or looseness).
