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Weird issue yesterday - Truck wouldn't "go" under WOT

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So yesterday I had a weird thing happen. I was driving at about 40 mph when I decided to do a quick WOT run just to see, and because I haven't done one in a while. When I did it, it seemed like the truck bogged down and couldn't get out of its own way. I couldn't tell if it downshifted because it bogged so bad. I did it 2 or 3 times just to see what was going on and it did the same thing each time. If I got on it gradually it had plenty of pep and pulled hard. On maybe the 2nd or 3rd time, it bogged like before but then all of a sudden took off. It was maybe a 3-4 second delay between when I pushed the pedal and it took off.

This morning I tried the same thing from about 40 and then from about 65. I gave it anywhere from 3/4 or full throttle and it downshifted normally and almost instantly and pulled really well almost instantly (complete opposite of what it did yesterday). The only different between yesterday evening and this morning was the transmission temp was around 180* yesterday and around 140* this morning (I'm due for a pan drop/filter change this week too).

Thoughts? Could it be the fuel pump going bad?

If it is the fuel pump, I have two questions. First, what size fuel line disconnect do I need and do you think two people could lift/slide the bed back?

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You could start off with just getting the fuel pressure checked. Then if it's "bad" you can move on to the fuel pump. I personally would just lift the side of the truck bed to gain access. The exact details of what size is what I do not have, sorry.

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+1 on testing the fuel pressure and add scanning for codes.

I lifted the bed on my GMT800 by myself... Removed bed bolts on the drivers side and loosened the bolts on the passenger side. Propped it up with a 2x4 until I could put jack stands under it.

If you have a second person it is MUCH easier!

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It can... depending on the severity and persistence of the condition.

It would be good to have a reader that pulls freeze frame data, that will give you other pertinent data when the code was thrown

It would be best to have a shop run a full diagnostics scan rather than just look for codes.

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