Fuel Starvation.

Schurkey

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Predictably runs good for 20 miles, then acts like it's not getting enough fuel?

I'd be looking at the fuel tank vent.

Easy test--loosen the gas cap, drive it more than 20 miles. If it runs good...you're likely pulling a vacuum in the tank as you drive, because the vent is somehow plugged. Loosening the cap provides a different way to vent the tank.
 

wb292

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Just in case some in the future searches this. I tried taking off the gas cap. No change. I hooked up the fuel pressure gauge I borrowed and it only made 6 PSI at idle. Turns out the new fuel pump is weak. I bought another new pump and installed it in the old sending unit. It made 15PSI at idle. Problem gone and truck runs great now. Pulls hard all the way to 4500RPM now. Drove it 30 miles with no issues.
 

Steve A

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Might try replacing the fuel pump relay if it's the original, contact points do quit sometimes. Something to think about anyway.
 

wb292

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The weak pump was from Orelliy auto. A Master Pro MPA 30029. Lasted a little over a week. I got a Delphi FE0115 from a local auto parts store. They only had a pump and not the whole assembly. So I put it in the old one. I had tried a new relay earlier this week but it didn’t change anything. Drove the truck down to Bullhead City today and it pulled the mountain back up easily in 5th gear. It’s a 6% grade for 12 miles. Very happy with the Delphi. Hope it lasts.
 

Schurkey

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I have a 91 K1500 RCSB with a 350 and a 5 speed.

bet the fuel pressure regulator spring is broke or diaphram torn.
If the spring was broken, a different pump would still have low pressure.

If the diaphragm was torn, there'd be gasoline pouring out, over the intake manifold.
 
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