Some back story on it. We had a nasty cold spell (ie -20 some mornings for about 4 days) a couple years ago. Prior to that the truck ran perfect other than the usual GM rust. After the 2nd day on the way to work the truck all of a sudden went into reduced power mode and drove the last 2 miles to work at 15mph. At lunch went out, CEL was still on, but truck ran fine and would rev up over what it wouldnt than morning. Drove fine on the way home even with the CEL on. Code came up evap leak. Cleared it and no CEL for a few weeks. Came back on, truck running fine though. Same code. Was warmer out so replaces the evap canister and valve. Still had the CEL pop up from time to time. Also ever since it went into reduced power, when I fill it up, the pump wont shut off before shooting some gas out the fill opening.


Well in late 2020 I got my moms van after she died. Been driving that more than the truck, but the truck ran fine up until this week. Has a new battery from this summer. Last month I took it out only to have the rear gas tank strap break. Replaced it and the truck sat for 3 weeks. Took it out the other day, and it now has a really loud quick random 1-2 sec hiss that comes from under the truck from the tank area. My question is would this be a symptom of the pump going on these trucks? Or would the evap system still be causing problems? Possible the cold snap damaged a plastic line on top of the tank?

I doubt  cold snap damged anything.....we regularly see -30+ in February, and it doesn't break vehicles, but it may hi-light an issue with a sensor that is going.....start with pulling codes.

only codes were for evap leak. SOmeone did say there may be a plugged line and needs to be blown out. Trying to fix this without dropping the tank if possible.

It's probably related to the evap canister and solenoid.  Some years got line and vent clogged up if you drive in a dusty environment.  

I'd have to look into more to be sure.  Google will show lots of videos and forums on how to fix the problem.

Had an older chev did that:
I drilled a 3/32" hole  through the inner plastic of the gas cap....
No HISSS from fuel tank overpressure.