Mossyoakglock

iTrader: (1)

Truck is an 08 Silverado w/ 5.3L AFM with 93,000 miles.

So for the last few weeks my truck has a stumble/miss on the first cold start of the day after sitting overnight. Right when the truck starts the RPMs jump to around 1200-1300 and then will drop right down to around 500-700 rpm for about 2-3 seconds and then jump back to 1,000 and then idle down smoothly like normal.

After that the truck idles smooth, drives great, good throttle response, etc. No misses or stumbles throughout the day and still getting 17 mpg average (mostly city driving). If I'm running around town and starting/stopping multiple times it never does it. Even going somewhere, turning the truck off for a few hours and then starting it back up, it doesn't do it either, only on the first start of the day after sitting overnight.

Now, if I let the pump prime and sit for about 8 seconds and start it, it doesn't stumble and starts right up like it should. Goes to about 1200 rpms and then idle down smoothly.

The plugs and wires are new (maybe 10,000 miles on them), I run Techron through the tank every two oil changes, fresh oil and checked for vacuum leaks which I found none.

My first thought is that it could be the fuel pump loosing pressure after sitting overnight. i checked fuel pressure and all it good though. But before I go changing parts I wanted to get some opinions on what it could be. I don't have time at the moment but I am probably going to take it to my dealer when I have time in two weeks and have them diagnose it.

mjcotroneo

iTrader: (13)

Check the maf. See if it's dirty. If it's not stumbling after you prime it I would definitely check pressure on the rail before you start and while it's starting up after it's been sitting.

Mossyoakglock

iTrader: (1)

It's definitely not the MAF. I keep that clean and check it regularly. The problem is intermittent as well.

I'm wondering if it is the Fuel Pump Control Module. I know there was a recall on them but my truck was not involved ('09 models I believe while mine is an '08). I'm wondering that if it's getting wet when it rains and then drys out when we don't have any rain for a while and works fine.

FFDP

iTrader: (2)

I bet the dealership will say that nothing is really wrong and that because the problem is not there all the time and it only lasts for such a short time period it's nothing they can do to fix it. Also once they start it, they have to let it sit for another day to make it happen again, good luck tracing down the issue.

If anything, I'd pull the plugs and check condition again. If they arent delco plugs or arent Iridiums, then I'd start there. Platiums are junk. You could also clean all the coil pack connectors and re apply electronic grease to the plug wires.

Mossyoakglock

iTrader: (1)

Those were my same thoughts when taking it to the dealer. It's always convenient that they "cannot duplicate problem" when you bring it in.

Plugs and wires were replaced with OE. I'll probably check them all again just to see and add some more grease. I checked the driver's side since they were the easiest to get to so I just need to check the passenger side.