Not sure what to do about this...

The fuel lines on my Bronco keep running dry and I can't figure out why. By dry, I mean, the engine dies and then the line is completely dry from the carburetor all the way back to the gas tank. The line going down into the tank is even empty.

When this happens I'm left sitting for up to an hour, waiting for whatever is causing this symptom to go away, and allow me to start my Bronco up again. It seems that nothing I do can speed the recovery up. It just won't pull fuel out of the tank.

This seems to only happen after a lot of hours of driving, and when I'm out in the middle of nowhere on up and down dirt roads. It'll just suddenly starve, and that'll be it for about an hour.

I drive the Bronco EVERY day on the side, back and forth to work, and it won't happen for months and months until I go out camping and off-roading, and then it almost happens without fail, leaving me dangerously stranded since the only way to see if fuel is coming through is to prime the lines by turning the starter over and over and draining the battery.


Things to consider:
Temperature isn't an issue. It's happened from 60° to 100°. I very much doubt it's any form of vapor lock.
New fuel pump.
New fuel filter.
New gas cap.
New rubber fuel lines.
Metal lines have been vacuum checked for leaks.
Tank has been removed and cleaned and is as shiny as a mirror inside.
Gas level doesn't seem to matter, from full to low.
It usually (but not always) happens when going uphill. However, rolling back down to a level spot does not fix the problem.



Considering where I usually am and how far I am from help when this happens, this is very frustrating, and can be very scary. I simply cannot trust my Bronco any more until I figure this out. Any help would be greatly appreciated. Not sure what to do since whenever I talk to a mechanic about this, they look at me like a deer in headlights clueless.


Ideas?

 

Does it "gasp"air when you remove the cap? maybe rubber vent line is pinched? I had a '79 Merc Grand Marquis that always smelled of gas inside,later found the vent line pinched shut.It must be a vacuum lock to keep the line in the tank empty,unless the fuel pick up is plugged,maybe something floating over it from time to time and when suction is gone from line,it moves away until next time.I have heard of this before.Don't you just love these head knocker problems?At least with a rod hammering,you know what it is!lol.

 

Yeah, it's gasped for air a few times when I take the cap off.
Vacuum lock sounds like it could be a possibility (since I know the tank is shiny clean inside).

However, with vacuum lock, wouldn't it remedy itself immediately if I were to take the cap off? Dies, remove cap, suction of air, vacuum lock gone, should start right up shouldn't it? Why the hour long wait?


Yes, this one is definitely a head knocker, and as said, can be scary considering how far away from civilization I am when it decides to rear its head... I like the obvious ones!

 

Probably a fuel line has either collapsed in some of it extention or you are suffering from vapor lock. Try this: Replace the fuel tank cap and see what happens.

 

Fuel line is all good. I've checked it all for damage, and pressure tested all of the line from front to back. I would also think that problems with the fuel line would surface more often. Replaced the gas cap twice.
And I know it's not vapor lock. I've dealt with enough old vehicles to know what causes that. That's usually from a hot day, from metal lines, from the fuel pump forward only, etc.

 

Do you still run a mechanical pump? I know you said it is new. My question would be (since you say it usually requires extended periods of driving for it to happen) if you are running a mechanical pump, are you getting full range of motion on the armature. If a mechanical pump "short strokes" it can actually work almost in reverse and push fuel back towards the tank. We have all seen pumps that have had shims added to the armature. The other concern with a mechanical pump is a worn cam. Extended run time and long uphill climbs could be allowing a pump that is not getting a full range of motion to lose prime. The "hour wait" to get it going again could also be contributed to a pump not getting enough range of motion to prime the line with any efficiency.

Just some of my thoughts... for what they are worth. Had a '69 Cadillac (Superior Hearse/Ambulance) that had this problem... and believe me, if you think pushing a stalled Bronco sucks... try a sharp-cornered luxury car that tips the scales at 6900 pounds.

 

Hey GreyStreak,
Yeah, still the mechanical. Thought about switching it out for an electric, but can't find one that doesn't have poor ratings, or explicitly says it needs to be mounted lower than lowest point in the gas tank (impossible in a Bronco), ie. gravity fed.

That's an interesting theory though. Are you meaning that the lobe on the camshaft the fuel pump rides on is getting worn out? If that's the case, I'd have never thought of that, but it'd make sense. The cam's going on 30 years, so I imagine it's had its fair share of wear.

Lol, of course, I'd rather push a 7k lb. luxury car on pavement than walking 20 miles away from a 4k lb. Bronco.

 

Yeah, either that or the arm on the new pump is slightly out of whack. I actually had an 1/8" shim riveted to the arm on the NEW pump I bought for the Cadi. Solved the problem. I determined the flow problem by cranking the engine without the coil wire to the dist. in place and the fuel line to the carb run down into a clear container to see what kind of volume/output I was getting with each rotation of the cam.