What's the difference between these two fuel pumps?

[Old Henry]

I filled the gas tank clear full hoping that, if there is a leak anywhere along the fuel line, it would show up figuring that, if air can leak in, then gas has got to leak out. Nothing. Didn't help the bubble problem either.

[trainguy]

The video shows a lot of air getting in the line,one would think if it was cracked or rusted full line it would drip gas when not running.Being that you filled the tank above the pick-up should rule out the pickup .My guess is that the line has a crack or pin hole above the fuel level in the tank.Phil

[oldford2]

Professor,
As many have suggested you must have an air leak somewhere from the pump back. Bite the bullet and install a brand new line from the tank to the pump. Don't waste more time looking for it. Even if you find the leak you will have to replace the line anyway. Duct tape won't hold for long.
John

[Old Henry]

Surprise. When I went out this morning the clear tube was clear full of gas. It hadn't drained back down. And, when I started it up and idled it there were no bubbles.

So, that leads me to believe that the pump is working fine (as it has tested to do last week) and it is the air that is causing the problem.

Since I haven't been able to find any leak in the fuel line that lets air in I am suspecting that there is a blockage in the line that restricts the flow and creates lower pressure in the line causing the fuel in that vacuum to vaporize and send vapor to the pump that eventually disables it.

What I will do next is put a T in the clear tube and run tubing to a vacuum gauge to see if there is a vacuum being created in the line. If so, then I'll know there is a blockage and will have to clean the line out.

Make sense?

[Old Henry]

Originally Posted by oldford2

Professor,
Bite the bullet and install a brand new line from the tank to the pump. Don't waste more time looking for it. John

I loathe just "throwing new parts at the problem" without accurately diagnosing the cause of the problem and knowing exactly what part has failed and why. I've had mechanics do that with some cars and never go there again when they do. With sufficient testing and diagnosis I am confident that the cause of this problem and its solution will be found without just "throwing some parts at it." But, thanks for the suggestion. I have become frustrated enough to do just that in the past but have been sorry I wasted my money on a false solution. I'll continue my methodical diagnostic approach.

[B-O-B]

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I would be looking in the tank for debris blocking the pickup. When you filled the tank the incoming gas moved it away from the pickup. I seem to remember someone suggesting that you get a mighty vac or similar tool & pull a vacuumn on the fuel line.

[Old Henry]

Originally Posted by B-O-B

I would be looking in the tank for debris blocking the pickup. When you filled the tank the incoming gas moved it away from the pickup. I seem to remember someone suggesting that you get a mighty vac or similar tool & pull a vacuumn on the fuel line.

The Mighty-Vac was recommended by two to help diagnose any problem with the gas line. Such would require disconnecting the line at both ends and would not duplicate the conditions happening while the engine is running. Instead, I'm rigging my vacuum gauge to detect any vacuum in the line while the engine is running. There should be none at all. Any vacuum will indicate a blockage that must be cleared.

[Mike B]

Originally Posted by Old Henry

The Mighty-Vac was recommended by two to help diagnose any problem with the gas line. Such would require disconnecting the line at both ends and would not duplicate the conditions happening while the engine is running. Instead, I'm rigging my vacuum gauge to detect any vacuum in the line while the engine is running. There should be none at all. Any vacuum will indicate a blockage that must be cleared.

Hmmn...Mighty Vac...maybe get a sealed gas cap, then connect the MV to the line going into the pump..but don't pump too much (how much, I don't know) and see if the gauge stays steady/holds vacuum.

I would use the brake bleeding bottle thing they come with in-line as well, so you don't suck fuel into the MV.

I'd think 3 in/hg would be enough? Unsure how much would be required to start collapsing the tank.

[Old Henry]

So, I connected my vacuum gauge to a T in the flex line and drove it. Vacuum hovered around 1-2 cm Hg. Didn't seem a lot but I think it should have been 0. So, I may well have a blockage in the line.

Is there a particular gauge of wire that anyone has used to run through the fuel line that is stiff enough to push through but flexible enough to bend around the bends?

[ford38v8]

Henry, throwing parts at the problem... You know that the old lines don't last forever, and yours is now older than you are, so...?

Just for giggles, climb under and give a fresh cinch to the connections in and out of your electric pump. I don't remember if you have solid connections there or soft, but the soft in particular are prone to requiring attention after some time.

Your reporting that your mechanical pump's check valve is malfunctioning is also not surprising. Nothing could be simpler than to pop it open and replacing those check valves with alcohol resistant valves.

[Old Henry]

Originally Posted by ford38v8

Henry, throwing parts at the problem... You know that the old lines don't last forever, and yours is now older than you are, so...?

You're thinking my 47 Ford. This one I'm struggling with is the 51 Custom I'm trying to sell for the widow of my friend that owned it. Exactly the same age as me.

Originally Posted by ford38v8

Just for giggles, climb under and give a fresh cinch to the connections in and out of your electric pump. I don't remember if you have solid connections there or soft, but the soft in particular are prone to requiring attention after some time.

Again, you're thinking my 47 that has the backup electric pump. If such was on this 51 I wouldn't be having any problems.

Originally Posted by ford38v8

Your reporting that your mechanical pump's check valve is malfunctioning is also not surprising. Nothing could be simpler than to pop it open and replacing those check valves with alcohol resistant valves.

I have ordered a pump rebuild kit including the diaphragms and valves, flex hose, settling bowl gasket, and pump to carb fuel line to replace the aftermarket filter in there.

[Capt Kirk]

I don't know anything about this particular setup but as I was reading the thread it reminded me of a problem I had with a jet boat I had. All tanks had "dip tubes" coming down from the top of the tank. In my case, I had a small hole in the dip tube at about 1/2 tank. Once it got to the level of the hole, it would quit. Had to switch to another tank and I was good to go. Fuel line from tank to pump shouldn't have access to air. If the fittings are good, tight and there's no rust holes in this vacuum side then there will be no air. I think 51Merc might have the answer. There might be a pin hole in the dip tube that isn't preventing suction of the fuel but allowing a small amount of air. Just a thought

[Capt Kirk]

Old Henry. No, the vacuum shouldn't have been zero. You're raising a liquid from a low level to a higher level and that requires some measurable vacuum (suction pressure) if the pump is at the upper end of the line. If the pump is in or near the tank then it will be positive pressure (not suction). If you had a blockage in your fuel line your vacuum would have read the suction capacity of the pump. I don't know what the vapor pressure (point of bubbling) of gas is but I'd be surprised if an old car fuel pump could reach that. Air is being introduced through a leak.

[Binx]

Old Henry, if you filled the tank and the bubbles stopped, then 51 MERC-CT's tip may be something to consider.

Originally Posted by 51 MERC-CT

A possibility is an internal crack or rust thru of the pickup tube in the tank.
If you haven't done it already try filling the tank full to see if the situation goes away until the fuel level gets below the defect.

Capt Kirk is also correct. You will see a vacuum reading when liquid is lifted vertically.

Lonnie