1977 f250 460 fuel pump

[alwaysFlOoReD]

My tow rig had a problem with the fuel pump [stock carbed 460/auto] and I changed it with new just before heading out on a 300 mile trip. On the way back I had problems again. The pump went down to about 3 psi and wouldn't keep the truck running. After waiting for the tow truck for an hour I decided to do another test and it was up to 5.5-6psi. I started up and carried on home with no further problems. Now I'm worried about taking any more long trips. What could be the cause of this drop in pressure, especially since the pressure comes back with time. Heat? I don't have a book handy, is there a fuel filter before the pump? Is there a sock on the intank line that could be clogged intermittantly? Other ideas? I'm going to look at it right now so any suggestions on how to proceed are welcome.

TIA,
Richard

 

[alwaysFlOoReD]

I'm stuck on the side of the road again with the same symptoms. 1psi pressure at the carb inlet. This time it also could be lack of gas as I'm near e on the gauge.

Sent from my XT1032

 

[alwaysFlOoReD]

Well its not lack of gas. I was able to get it running and fueled up the front tank. Crapped out again after 10 minutes.

Sent from my XT1032

 

[alwaysFlOoReD]

So it seems like the pump works after sitting for awhile. What could cause a pump to slowly come back to putting out full pressure? I loosened off the gas cap so I could rule out a vacuum in the tank. I couldn't find a filter between the tank selector valve and the pump. Having two tanks would seem to rule out floaties in the tank, but not completely. Anything else I'm not seeing? Could it be some kind of heat related vapor lock? The rear tank is basically empty so I think I'll pull it down tomorrow to rule out floaties.

 

[RonD]

I assume it is a mechanical pump on the engine.

It could be bad or you are getting clogged or vapor locked lines(hot days).

What about the dual fuel tank solenoid?
They can restrict flow if failing

 

[alwaysFlOoReD]

It's a mechanical pump. Replaced last week when this problem originally showed up. I will check the line for the rear tank tomorrow when I drop the tank. How do I check if the selector valve is failing? And how does the vapor lock work? I would think that if a bubble formed in the line between pump and tank the pump it would get sucked out. Or does it form as fast as the pump sucks?

 

[RonD]

Fuel Pump can't pump air, only fluid so when air gets to the pump fuel flow drops to almost nothing.
If you let it sit the air will often get out the higher line, and let enough fuel seep in to clear the air when pump is started again.

Vapor lock is a long shot, but what happens is the metal fuel line gets heated up by exhaust pipe and gasoline inside starts to "boil", form air bubbles, when it gets to the pump, pump stalls because of the air.

Because this is a siphon system a small break in a fuel line can cause air to be sucked in along with some fuel., but you should smell gas when engine is off, unless leak is in a fuel tank line.

usually that year dual tank solenoid would just have 1 line in form each tank and 1 line out to fuel pump.
It uses 12volts on, 0 volts off
Inside there is a slider with two passages, and a spring.
With 0 volts the spring pushes slider so "tank 1" is connected to Out line, when 12volts are applied the solenoid pulls the slider over against the spring and "tank 2" is connected to Out line.
If slider is sticking or not moving all the way there can be limited flow.

 

[alwaysFlOoReD]

I've seen a reference to vapor lock being common to these years in RV's. Apparently they went to in tank electric pumps to cure while still carbed. I will check routing of fuel line and see if I can insulate.

Selector valve is as you describe. I may check that if I get energetic.

Thanks

 

[alwaysFlOoReD]

I was under the truck today. I found a rubber line from the tank to the steel line was missing a clamp - fixed. Checked operation of tank selector valve - working. Blew out the line going from the valve to the rear tank. Then finally dropped the rear tank. I found the sock had fallen off the suction tube. Finally retrieved it from out of the tank and used a spring to hold it in place. It was loose and I didn't know if it's supposed to be glued on or what so the spring seemed like a good idea. We'll see.

The sock and claw with flashlight I used to remove it from the tank;

I need to use the gas in the front tank before i drop that one to check it too.

 

[alwaysFlOoReD]

Today I took the truck out and deliberately ran it dry on the rear tank. Had problems starting when I switched to the front tank. Got it running after sitting for an hour. Went and filled the rear tank and fuel delivery problems cropped up again after running down the road go a mile. I'm leaning to a heat problem because it will start fine after sitting for an hour. I'm getting less than 3 psi when hot. It gets to 7 psi after sitting.

Sent from my XT1032

 

[alwaysFlOoReD]

I took the fuel selector valve out and bench tested it. The solinoid is sticky and doesn't work correctly most of the time, but does work sometimes. I'm replacing with a manual selector.

 

[alwaysFlOoReD]

Its not the fuel selector valve either. I swapped on a manual valve and have the same problem. Going to swap out the fuel pump tomorrow morning under warranty.

Sent from my XT1032

 

[alwaysFlOoReD]

Swapped in the new fuel pump and it seems to be good. I drove double that distance that would normally show problems and the truck hasn't died. Knock on wood.

Sent from my XT1032

 

[RonD]

Good work

 

[alwaysFlOoReD]

Thanks. It's frustrating doing all that work to prove a new fuel pump is bad. I can see where people will just warranty something several times rather than test the system. Which drives the prices up and quality down in a price driven marketing system. And I'm guilty of the lowest price syndrome too, but am trying to buy quality first in the last few years. It's a hard habit to break.