Question:
Are the valves moving at all at the initial crank/start up say the first second or two or are they dead still.
GM dealers have the special tools to test the VLOM so I’d say next step would be the dealer either with the truck, or to see if they can test the VLOM for you. The GM test equipment has a solenoid control box and shop air supply. They pump 30psi to the VLOM and then use the control box to run each solenoid to test their function.
When in V8 mode, the VLOM solenoids are in the closed position thus not letting oil flow to the lifter locking pins. When the solenoids are turned on, oil is sent to the lifter locking pins and disengage the pins to allow the lifter to “collapse.” If you have a VLOM solenoid sticking ON, that would explain why it is collapsing the lifters thinking it is in V4 mode.
If you aren’t worried about testing the existing VLOM, its ~$300 – 500 at a local dealer p/n 12571609. You will need intake gaskets as well ~$120.00 p/n 89060413.
Me? It wouldn’t hurt to have the VLOM tested if you already have the truck apart yourself.