Fuel Flow/Pressure Drop

In a separate thread I posted about replacing the pickup tube as I was having fuel delivery issues. Well I thought I had it solved but it still is not working correctly. All the lines are new and leak free, have tried 2 rebuilt stock pumps and now I am on a NOS Carter civilian pump. The fuel filter is staying full for about the first 8-10 miles then it slowly empties out so the truck will barely run, you can see just a light spray of fuel going into the filter. So I removed the filter and installed a in line fuel pressure gauge, from a cold start it's showing a steady 5psi. After about 6 miles I checked and it dropped to 4.5psi, after about 8 miles down to 3.5 psi and arriving back home about 12 miles it was jut below 3psi. I let it cool off for a few hours, re started and it shows 5psi, just don't understand what is causing this, hopefully some experienced folks can provide some clues.

Thanks

I think the stock pump puts out about 7-8 psi.

Remove your fuel tank cap and try again. If the vent in the cap is stuck or it is an unvented cap, you could be pulling a vacuum on the tank.

The cap is vented and I have the vent line on the side of the fill neck to "open air" for extra venting, I also tried it with the cap off, no change, this is both before and after I replaced the pickup tube. The manual says 3.5 to 5.5 psi. I also blew the lines with air and no obstructions. Appreciate the reply.

Did you replace the fuel line between the frame and the engine on the drivers side. It's about a 6" rubber line.

When I am troubleshooting fuel problems, I will pull off that fuel line and run the engine side to a 5 gal can to eliminate the fuel tank. Strap it to the brush guard and you can drive with it.

The fuel pump should fill up a 16 oz water bottle in around 30 sec if pumping enough fuel.

I did replace the rubber line at the frame/engine, all the lines are new. So I went with plan "C" and installed an electric fuel pump. I went with a mil-spec Facet 24v 4-5 psi unit, I installed it on the inside of the frame across from the transfer case where the fuel line junction was. I drilled/tapped the frame for the mounting bolts and added spacers so the pump would clear the brake and wiring harness. I also took the check valves out of the stock pump so the fuel would free flow but still have the look of the stock pump. I also put a heat shield cover/sock over the metal line going into the stock pump, it seemed to be getting quite warm being close to the exhaust manifold. I now have a steady 4.75 psi at the carb and it runs great.

Glad you got it fixed!

I'm having the exact same problem. Truck starts out with plenty of fuel and after a few miles, the filter is almost empty. I also noticed when removing the fuel line at the carburetor, there is a tremendous amount of pressure in the line. I also thought I had it fixed after rebuilding the fuel pump and replacing all of the lines from the tank. The truck has a new tank. I dropped it and inspected for anything that might be clogging the line, but that was negative. I swapped out with another fuel pump and got the same results. I'll probably go with an electric pump as well, but I still want to know what is causing the problem and why there is so much pressure on the fuel line at the carburetor when this happens.

After searching the site more and reading through bunches of old posts, vapor lock seems the likely culprit. It had crossed my mind, but I dismissed it. I also read through the manual concerning troubleshooting the fuel system. I've been running Shell 93 octane non-ethanol fuel. I thought that would be a good choice, but maybe not.

I had the same results,when removing the output line at the pump I would have pressure but no or very little fuel in the filter. Running 91 non ethanol here.

Just curious.
The trucks that are having trouble with possible vapor lock. Do you still have the fan shroud installed?

Mine is installed.