So, I was going down the interstate at 65mph, and all of the sudden the truck starts slowing down, and every time I tried to accelerate, the rear end of the truck kept jumping and losing speed. It almost felt like the rear wheels were trying to keep up but couldn't. I pulled over, sat for a while, checked trans fluid and oil, and everything seemed fine. I decided to start the truck back up and try to get to the next exit which was about a mile and a half away. Got up to 65 fine and was able to get to the next exit with no issues at all. Ended up driving home through town, which took around 30 mins, with still no issues. I'm thinking there's something wrong with the rear end. The only other thing I can think of is there's something wrong with 4th gear, but I figured the issue would be consistent if that was the case. It's a 1990 5.8 auto with the 4x4 c6

 

More likely a fuel delivery problem. Click this & read the caption:

(phone app link)

 

You're right, it is a fuel delivery issue. I had an aftermarket locking gas cap, because I had lost my original, so we figured the truck was getting vapor locked. We changed it to an OEM gas cap, and the issue seemed to disappear, and the truck ran better. All the way up until a couple days ago. The truck got heated up and did it again, and it started running a little worse. We're thinking that, for some reason, it's flooding itself.

Another note: Whenever the truck does the whole jumping issue, when you press on the gas, it starts slowing down and dying, but if you let off the gas then it'll cruise and you'll be able to keep going after a little bit.

 

The fuel cap can't cause a fuel delivery problem. EFIs can't experience vapor lock.That sounds like low fuel pressure; either due to running the tank dry (but then it wouldn't clear up just by driving), or debris in the tank blocking the fuel pickup, or a weak fuel pump, or a rupture in the FDM valve head as shown here:

(phone app link)


Read the PREVious 3 and NEXT several captions for more info.