351 Cleveland losing prime

[oldmountainfarm]

72 Mach1 is losing prime in the gas line. I replaced almost everything in the restore. New gas tank and lines, new mechanical fuel pump and carb. I cleaned the sending unit but didn't replace it, I have a new one now to put in but I just don't think that is my problem. I got it cranked and it ran like a charm. Try to crank it a week later and there is no gas in the lines. I took the mechanical fuel pump off and put an electric one at the tank. Get it cranked and it runs fine, try to crank it a week later and there is no gas in the lines. I can take the line lose from the fuel pump and suck the gas through and get it flowing and hook it back up and it runs fine. Gas cap on or off, same thing. It's finished and ready for the July 4th parade but I'm scared to death it's going to just cut off in the middle of the parade because of losing prime and as many Chevy's there are in the parade, it would be quite embarrassing. Any suggestions?

 

[Manu Mach1]

Hello,

Not sure we originally have a checkvalve somewhere in the line?

Manu

 

[72HCODE]

ummm,, i'm not sure you even have a problem here...

if you let the car sit a week, there is some drain back but also the fuel bowls evaporate.

usually if i don't drive for a week the procedure is, get in car, crank for about 20 seconds, then press pedal to reset fast idle and reset choke, then crank, usually it fires up after a couple of seconds.

are you saying no matter how long you crank after a week it refuses to pump fuel before you need to reprime the lines?

it seems like the car is then fine again unless it sits for a week. It isn't a modern car where you turn the key and the fuel pump kicks on pressurizing the line to the fuel injector.

the carb is going to be stubborn after sitting for a few days, but after you get the fuel to the carb and the bowls refilled it starts right up. if you daily drive it there shouldn't be a problem.

Hot starts or fuel boiling is another story.

 

[oldmountainfarm]

No, it will not start unless I take the line loose from the tank and suck it to get it flowing again. then hook it back up to the electric fuel pump and then you can hear the difference in the fuel pump pumping actual gas instead of air. I've got several of what you call non modern cars and when you take the line loose from the fuel pump, there is gas in them. There should always be gas in the line between the tank and the fuel pump unless you run the car out of gas.

 

[Mustang Dynasty]

72 Mach1 is losing prime in the gas line. I replaced almost everything in the restore. New gas tank and lines, new mechanical fuel pump and carb. I cleaned the sending unit but didn't replace it, I have a new one now to put in but I just don't think that is my problem. I got it cranked and it ran like a charm. Try to crank it a week later and there is no gas in the lines. I took the mechanical fuel pump off and put an electric one at the tank. Get it cranked and it runs fine, try to crank it a week later and there is no gas in the lines. I can take the line lose from the fuel pump and suck the gas through and get it flowing and hook it back up and it runs fine. Gas cap on or off, same thing. It's finished and ready for the July 4th parade but I'm scared to death it's going to just cut off in the middle of the parade because of losing prime and as many Chevy's there are in the parade, it would be quite embarrassing. Any suggestions?

like your torino-lots of room and bet w/the whole body-handles better than the no rear 71-3 mustangs though would take a lot to rid me of my 72 stang covertible-cotm march for next year. Mustang dynasty-mikke

 

[oldmountainfarm]

I'm trying to sell the Torino or trade it. You wouldn't believe the crazy offers I've gotten.

 

[CZ-75]

When you disconnect the fuel line from the pump (the line coming from the tank), does it start pouring out freely, or do you really have to suck on it to get it flowing?

If the latter, there has to be something preventing gravity from doing its job. I recently replaced my fuel pump, and as soon as I took the fuel supply line off the pump, gas started pouring out, and I had to plug it with a golf tee to keep gas from coming out.

There is either a vapor lock, that sucks gas back toward the tank, or maybe a crack in the fuel line that lets the line become empty over time, or some sort of clog in the works.

It very well could be that you have a cracked or otherwise messed up sending unit

 

[oldmountainfarm]

That's what I keep saying, when you take the line lose from the pump it should be full of gas and like you say, start pouring out. It doesn't matter if you drive it every day or once a year. My electric fuel pump is right next to the tank so the fuel line to the pump isn't a foot long. The fuel line is not cracked from the tank to the pump and if it was, it should leak till the tank is dry. When I first asked this question quite some time back, I got all kinds of responses from cracked fuel line to bad fuel pump. I replaced all the lines from tank to carb and a new mechanical fuel pump on. Still the same problem. I have the new sending unit, I guess I'll try it. I was just thinking there was some kind of vacuum problem. Like the vent line was clogged or something. For gas to go out of the tank, air has to get in, correct?

 

[CZ-75]

Yes, you are correct. BUt I thought you said the same thing happens with the fuel cap removed. That would eliminate any vent problems by creating a huge opening!

But if your sending unit was cracked inside the tank, allowing gas to vaporize / leak into the tank, then you might lose your prime. And if it's a small enough leak, your electric pump may be able to componsate for the "hole in a straw" syndrome.

 

[oldmountainfarm]

That makes perfect sense. I will try the new sending unit. Thanks! It's always good to discuss problems and get other peoples views.

 

[Don C]

How about fuel level, does it do the same thing with a half to a full tank? Because our sending units are located near the bottom of the tank, with more than a half tank of fuel the pick up tube would be completely submerged, with no air leaks possible in the sending unit/pickup.

Your electric fuel pump should be able to pull some air, anyway, as the mechanical pump can. Anyplace where the fuel line is higher than the tank would be a likely place for air to leak into the line.

Because the mechanical fuel pump has a check valve in it, fuel should not return to the tank, even if you have something that is causing a vacuum in the tank, like expansion/contraction and plugged vent line.

 

[oldmountainfarm]

Interesting. It does it no matter how much fuel is in the tank. Whatever the situation is, when I remove the fuel line from the tank to the electric fuel pump, no fuel will come out the tank. It should be pouring out shouldn't it? Gravity would make it come out right?

 

[CZ-75]

Do you have a filter in the line? Some filters are unidirectional

 

[oldmountainfarm]

Yes but the filter is going in the right direction and that still doesn't explain why when I take the hose off the electric fuel pump to the gas tank which is only a foot long with no other connections or filters and hold it down to the ground, no fuel comes out. I can suck on it and hold it down to the ground and gas just starts pouring out.

 

[caspianwendell]

If you can suck fuel out of the line, the fuel pump should too...gotta be a problem there

 

[Don C]

I would say the sock is gunked up with varnish, or maybe water, that keeps the gas from flowing freely. But, this doesn't explain where the fuel in the line is going. Either pump should be able to suck harder than you can, if it's in fuel.

Are there any inverted "U"s in the line that could be getting and holding an air bubble?

Do you have a bypass fuel regulator or fuel filter with a return line? If plumbed wrong they can circulate air instead of fuel.

 

[mach1chick]

Don't know if this will help you but I had a similar problem on a car and found a loose clamp (didn't look loose and I thought sure I tightened it) that was allowing the fuel to drain back into the tank. Now when I do any work on the fuel system, I go thru the clamps several times...with a socket, not a screwdriver.

 

[CZ-75]

Any updates?

When you suck on the line and get fuel, does it keep pouring out, or does that just get enough fuel in the line to prime the pump?

If it keeps pouring out, then that kind of eliminates a clogged sock. And would point even more to a "hole in the straw" somewhere inside the tank, which eliminates the syphon effect, which gets gas to the pump before it starts working.

 

[NitrousVa]

I wonder if the vapor line is clogged? Have you checked that? It's the line over the transmission, going to the vapor/charcoal can.