Hi everyone,
Last year I was having some issues with my Bronco running dry on fuel. I thought it was vapor lock, but from what I've read, that usually occurs between the fuel pump and the carburetor.

It happened more when it was hot out, but I've even had it happen as cool as 85° outside, so not necessarily blazing hot or anything. The engine would start to surge and then sputter and die.

When I would get out to check it, the fuel line was completely dry from the carburetor all the way back to the fuel tank. It could also take upwards of a half an hour to an hour before the fuel lines would empty of vapor and I could get the engine started again. Before that, if I tried to start it (with the fuel line at the carb disconnected), I'd just see puffs of vapor coming out of the line.
Also, the fuel line doesn't go anywhere near the exhaust manifolds. It's visible in this picture running from the fuel pump (lower front driver's side), up over the valve cover, and into the carb.



I never once had this issue with my stock 1bbl carburetor for all the years I'd owned it, and it's only started occurring once I installed the Holley 390 4bbl.

The other day I was out working on my engine and noticed that when I converted my intake setup to a 4bbl, I capped off the charcoal canister. The stock 1bbl had a bowl vent that would vent excess fumes from the carb down to the charcoal canister. There's also another line that exits the canister and runs all the way back to the gas tank.

Since the 4bbl didn't have this provision, I just removed the line, left my charcoal canister in place (along with the line to the tank), and capped off the port that originally ran to the carb.

Got me to thinking, when it's hot out, is my gas tank filling up with fuel vapor? And since the charcoal canister is capped off, it can't vent out through the canister, and ultimately has to go somewhere, so it vents out the actual fuel line to the engine? I dunno if this is how this system works, or if this cap is doing nothing. It doesn't happen often enough for me to just take the cap off and test my theory, so I'm curious from someone who understands how the system works.

Going camping next month and am going to be out in the middle of nowhere. Don't feel like sitting on the mountain side for an hour while the engine cools back down like last year. Especially since this year, there'll be family there to impress.

Thanks for any information.

 

Lol your engine look just like mine, well mantained but ugly and dirty as hell...

 

I take that as a compliment.

An engine's true beauty is in how it performs.

 

To answer your question, yes, the fuel tank vents to the charcoal canister, which stores the gases (HC) until the truck is started. The carb then pulls in the stored HC from the canister and burns it.

 

Interesting, and good to know.

So, effectively, what I've done, is still allowed the tank to vent up to the charcoal canister, but then plugged off the line that sucks those fumes into the carb? If that's the case, could this be a reason why the lines are running dry?

It would seem that if this is the case, my tank is filling with gases, but instead of being able to vent out through the carb, the pressure starts to push it out the only exit, which is the fuel line to the carb. Would this be correct?

How would I remedy this?

 

I am not sure of the setup on your year Bronco, heck I am speaking of mine from memory, but in general, there should be 2 lines from the canister to the carb. One goes to the float bowl vent which is meant to collect vapors from the gas just like the line to the tank. Then there is a vacuum line that is attached to ported vacuum either at the carb or on the intake. This leads to the purge valve which decides when to release the stored HC in the canister to the engine for burning. I can't remember, tho, on yours if the purge valve is integrated with the canister or if it is separate.

If all you did was plug the line from the float bowl to the canister, and everything else is hooked up as it was, then this should not cause your problem IMO. But you could just hook that line up to your new carb, it has a nipple on the bowl vent, doesn't it?

 

Currently, it has a breather valve on top of the gas tank. Looks like this:



A line runs from that to the front, where it goes into the charcoal canister, that looks like this:



Going from memory from here on out too...

From there, there was a big line that ran up to the front of the carb on my old 1bbl. The Holley 390 doesn't have a fitment for this at all. The fuel bowls just vent up into the air filter housing, which I imagine allow them to get sucked back in.

It did have this purge valve in the system for a while that ran to the PCV, as well as a ported vacuum switch on the front. It's since been removed. Looked like this:





Right now, there is only the line from the tank to the charcoal canister. There's no line at all exiting the canister. No line running from the canister to the carb, or the purge valve. It's just blocked off.

 

Forgot I had this picture. This clearly shows how it was hooked up:



About the only line (minus the heater hoses) in the picture is from the charcoal canister (lower right) that runs up to the top left. A smaller line splits off and goes up to the Purge Valve in the top left, which then goes to the ported vacuum switch at the front of the engine and to the PCV.

The big line then continues to go to the top left where the carb was sitting (ends right above the opening on the intake), and has some sort of one-way valve on it.


All that's been removed and capped off at the charcoal canister.

 

Well I would guess that your fuel tank is not vented properly. This would cause the system to pull a vacuum on your tank which would lead to sporadic fuel starvation. Of course this assumes a sealed system which it was in 1981 but may not be now. I would just unplug the carbon canister and leave it open to air and see if that fixes the problem or not.

 

Sealed system as in no leaks? Yeah, I pressure tested it a few months ago and it was tight as a drum.

Which system are you referring to that would pull vacuum on the tank?

I'll try leaving it uncapped for now and see if it helps. It seems to be more so when it's hotter, or steep inclines, (although it's happened on normal roads after hours of driving) so unfortunately I have no idea when or if it'll happen again. Not something I can replicate easily, but definitely am trying to figure out how to prevent it.

 

The fuel delivery system, ie, fuel pump, is pulling a vacuum. The purpose of a vent is to allow air in to replace the gas that is being pulled from the tank and burned in the engine. If you don't let any air in, you get a vacuum.

 

Oh, gotcha. Okay, that makes sense and would explain a lot.

My last question would be:

If I created a vacuum in the tank due to capping off it's inlet, wouldn't removing the gas cap immediately relieve it? Why would it take a half hour to an hour for the pump to pull any more fuel from the tank? Excess pressure still in the line from the tank to pump? Pump temporarily over exerted?

Just like to understand these systems 100%.

 

That part is interesting. If the same thing happens (just puffs of vapor coming out of the line to the carb, with carb disconnected) AND you have the gas cap off, that would point to another problem.

Mechanical fuel pumps are good for pumping liquids. They are poor for creating a vacuum to pull vapors. A fuel pump with worn rubber valves inside has more trouble doing this. Or if a piece of rust or crud is stuck into either the pumps input or output valves, holding the valve open a bit.
For old vehicles, an inline fuel filter in series just ahead (the tank side) of the fuel pump is a good idea.

I ran into a really nasty problem back in the 70's that drove me up a wall. Working on my car in an apartment parking lot every day after work, wondering if I would make it all the way to work, or back home again at night.

I finally figured it out. The gas tank was not coated properly internally. Rust built up and started to slough off the tank walls, and with enough running, started to coat the pickup screen in the gas tank. So it got real dicey to drive, die out when stepping on the gas, quit, trouble restarting while people are honking at me trying to go up a bridge, etc. Awful. But the next morning, everything was fine, would make it within a mile of work before it would start problems again. And at night on the way home, could make it part way home before it would show up again.

When parked long enough, enough of the crud that got sucked up against the pickup screen fell off, and floated away, allowing it to run great... till it sucked enough crud against the screen again.

 

Yes, it should relieve it immediately. You may have other fuel delivery problems as Torky2 pointed out.

 

I don't know then, and am at a loss. When this first started occurring, I pressure tested my entire fuel system and it was top notch. I also removed the tank to clean it and check for any junk in the tank (since someone suggested that at the time), and the interior of my gas tank is like chrome. There were a few random bits of debris in the bottom (looked like tiny rocks), but they didn't even float and certainly weren't big enough to block anything.

This is a horrible problem when it occurs because I have to wait a very long time before the vehicle will even start again, and it can happen after hours of driving so I can be in some pretty far away places by then. The worst was honestly on the side of a mountain at 8000 feet with no civilization for 20 miles. It was over an hour before it started again. Fuel lines remained dry all the way back to the tank.