First start of the day stumble/stall issues

[Coytet]

G'Day from Australia, I have a 2022 1500 5.7L, I picked it up wit 15Kkm on the speedo, since I've had it ( now 44Kkm) first start of the day it will start rev up to 1400rpm then as it starts to settle it will drop to around 950 rpm and catch it'self back up to 1400 rpm and then do it again (second time not as bad), if you drop it into drive or reverse prior to the stumble it will sometimes stall or have a massive stumble with rpm drop to about 600 rpm before it recovers.
It has been at the dealer over here for a total of 22 weeks.
So far they have replaced -the fuel pump-full set of injectors-PCM-throttle body-and swapped out the 4 oxy sensors.
Now they have advised that it's because I'm using 95 or 98 RON fuel and I need to go back to 91 RON which is the equivalent of US 87.fuel
Has anyone had the same issue and had a fix for it?

 

[tron67j]

Thinking maybe vacuum leak or dirty throttle body are possibilities.

The truck is designed to run on our 87 octane fuel or your equivalent, using a higher octane fuel is mostly a waste of money as long as the fuel you use is good quality. Could the computer be trying to adjust to the unexpected octane, possibly. Run it all out, put correct octane fuel in and see what that does before you start doing other things. Eliminate that variable first, then if not better check all your vacuum lines carefully for leaks. If nothing found try cleaning the throttle body with cleaner. If still a problem maybe the idle air control valve but don't know how to test that. Not expensive to replace, at least here in the states but try other things above first. Last thing in my head is maybe one or more broken header bolts causing a leak or a cracked exhaust manifold. Maybe someone else will chime in with other thoughts. Good luck.

 

[Bigskyroadglide]

I'd start with checking the intake manifold bolts. 108 inch lbs, don't over Tighten. Remember you are tightening into plastic

What you are describing sounds like a vaccum leak that goes away as the engine becomes warm.

 

[CanuckRam1313]

^^^^ THIS!!!

Great starting point as our 5.7 intakes are notorious for loosening after time.

Make it part of your every 6 month checks

 

[chopperman1]

I know you said it's a new fuel pump, but it sure resembles fuel pressure bleed down. Try this.... On your first start of the day, turn the truck "on", do not start it, wait about 15 seconds, then start it. If issue resolved, your pump is bleeding down, means bad check valve. If not, at least we ruled that out. Intake manifold would be my next guess as mentioned above.