I wasn't sure where to put this so please move if needed....

This is on a 2008 Silverado w/ 5.3L.

So I am having a bit of a stumble at first cold start of the morning and I’m trying to diagnose what it could be. It only used to do it maybe 2 times a month but for the last two weeks it has done it every morning. It will start perfectly fine go up to about 1,000 RPM and then drop to about 700-800 RPM and sort of miss/stumble and then jump right back up to 1,000 and slowly idle down like normal.

I have tested fuel pressure and these are the readings I have taken. They have all been at the first start of the morning after the truck has sat for 10-12 hrs and at the same time each morning (6 am).

1. Thursday morning, Key On/Engine Off – straight 60 psi, no drop in 1 min

2. This morning/Friday morning, Engine running/idle (did not let the pump prime, turned it over right away) – straight to 60 psi then drops to 46 psi and stabilizes. No drop in PSI in 1 min.
A. Just to do it i left the gauge on for 2 ½ and 5 minutes and there was only a 1 psi drop in 2.5 and 5 minutes.

Yesterday after work I decided to check the fuel pressure and it was bouncing between 50-54 PSI and dropped 3-4 PSI in 1 min after shutting the truck off. After 15 min it had dropped 30 psi.

It only does this on the first cold start of the day. I can drive to work and let it sit from 7am – 4 pm and it won’t do it and running around town and starting multiple times a day it won’t do it. Other than first start in the morning the truck fires right up and runs great with no codes, etc.

Two things did happen that were odd though. First is that the fuel gauge occasionally will creep up once started. Not much but enough to be able to see it move. And a few weeks ago I wanted to get on it a little on a backroad and when I pushed the gas pedal, it didn’t go anywhere and sort of bogged down. I pushed the pedal about half way down and for about 3-5 seconds just bogged and didn’t go anywhere and then all of a sudden it took off like a bat outta hell.

My thoughts are either leaking/clogged injector, fuel pump or one of its components are going out or the fuel pump drive module. But based on the fuel pressure readings I don’t think either of those are the problem. I do have an intune but I am running the Modified Stock tune with just AFM disabled. I ran Techron through it maybe 2000 miles ago and plugs and wires are 8-9 months old (used AC Delco plugs and wires).

The only other thing I can think to do next is pull the fuel rail, prime the pump and see if any of them are leaking. I can also hear the pump prime for 2-3 seconds.

If I let the pump prime and leave the key in the accessory position for 5-10 seconds the truck fires right up with no stumble/hesitation.

Thoughts? I had it with the dealer and everything checked out ok for them.