Sucking air/Losing prime

[Selahdoor]

I’m sorry but you’re wrong here, take your fuel pump off, and blow through each end. There’s no check valve.

Blowing air through is quite different from pushing fuel through. Especially at the pressure you can muster by blowing through with your mouth. Try it again with a big blast from an air hose.

Better yet, get out your brake bleeder or something similar and try to push fuel backwards through the fuel pump at a pressure of 4 pounds or more.

Air will easily leak, where a liquid will push the valve closed. It's likely it will leak even with a blast from an air hose, but you might actually notice a bit of a slowdown of the flow.

It works the same as many other diaphragm pumps, inlet is uncovered on the intake stroke, and output is uncovered during the exhaust stroke. Please take one apart and post pictures so we can put this to rest

The very definition of a check valve...

It is IMPOSSIBLE for our mechanical/diaphragm fuel pump to work without some method of preventing backflow. Whatever method is used, it amounts to a check valve...

 

[Thewespaul]

There’s no valving, just orfices at different heights. Yes it checks when the engine is rotating, but once it’s not there’s nothing stopping the fuel from draining back if the arm is not on the lobe of the cam. Not only have I blown through pumps with them out of the truck you can visually see the fuel drain back with clear lines.

 

[Selahdoor]

Believe me, I am completely willing to be wrong on this.

I just don't see how it can be physically possible.

I do hope someone has an old one they can tear apart.

HEY! @MacRobb! You got the time to do this? Good video idea too!

 

[PROFG]

After i was shot down for same observation, decided to wait until I could tear one apart. Could see it working if piston type with ports like 2 stroke engine but diaphragm??

 

[snicklas]

If I remember correctly, how a diaphragm fluid pump works, and I think there is where the confusion is coming from..

In a diaphragm pump, it works more like a reed valve setup, where you have a "flap" that covers the port, and depending which way the diaphragm is moving, it pushes one flap open, and one flap shut. So to help picture this, on the inlet side of the pump, from the tank, the flap would be on the same side of the inlet as the diaphragm. On the outlet side of the pump, the flap would be on the outside of the passage from where the diaphragm sits. So in the intake stroke of the diaphragm, the "vacuum" created in the pump would pull on both flaps. Since the flap on the inlet side is on the "inside" of the passage, the flap will open, allowing fuel to enter the body of the pump. This same "vacuum" is acting on the flap on the outlet side of the pump, but since it's on the "outside" of the passage, the flap is pulled tight against the port. Both of these flaps have the ~5psi of pressure acting against them when the pump is moving. Now, for the other half of the cycle, as the diaphragm switches from the pull (intake) stroke, to the push (exhaust) stroke, the diaphragm now pushes on the fuel in the body of the pump. The flap on the inlet side of the pump, is now being pushed against the inlet port, closing the inlet port. The flap on the outlet side is now pushed open by the the fuel the diaphragm is now pushing against, and the fuel is pushed out of the pump up the the fuel filter.

I think where the "picture in your head" is getting fuzzy is what a check valve is. An actual check valve (like the one that Wes is selling) is a device that has a positive sealing design to it. There is a ball and spring that is calibrated to a certain pressure (even if it .5 or 1 lb), the spring pushes the ball closed causing a seal. No flow happens until the spring seat pressure is overcome. Once you have sufficient pressure to push the ball against the spring and move it out of the way, there is no flow. The reed valve flaps (which are some kind of "rubber" in the lift pump) are just that, a flap. There is no return spring to push them shut (other than the weight of the fuel in the lines and the pump body). So a new pump with healthy flaps might act as a check valve. But as the flaps age and get more pliable as they age, the fuel can sneak past them, since there is nothing pushing them closed like true check valve.

Think of it like this. If a semi is moving forward down the road, the pressure (wind moving past the truck in one direction (the diaphragm pump moving in the lift pump scenario)) will push the rubber mud flap away from the tires in front of it. But, if the truck is not moving, the breeze blowing (now the wind is moving, not the truck (diaphragm pump not moving in the lift pump scenario)) the breeze (fuel) can move the mud flap where ever it wants to. It can push it away from the tire, or bang it into the tire, since there isn't the wind moving past the mud flap to keep in in one position.....

 

[IDIBRONCO]

If I remember correctly, how a diaphragm fluid pump works, and I think there is where the confusion is coming from..

In a diaphragm pump, it works more like a reed valve setup, where you have a "flap" that covers the port, and depending which way the diaphragm is moving, it pushes one flap open, and one flap shut. So to help picture this, on the inlet side of the pump, from the tank, the flap would be on the same side of the inlet as the diaphragm. On the outlet side of the pump, the flap would be on the outside of the passage from where the diaphragm sits. So in the intake stroke of the diaphragm, the "vacuum" created in the pump would pull on both flaps. Since the flap on the inlet side is on the "inside" of the passage, the flap will open, allowing fuel to enter the body of the pump. This same "vacuum" is acting on the flap on the outlet side of the pump, but since it's on the "outside" of the passage, the flap is pulled tight against the port. Both of these flaps have the ~5psi of pressure acting against them when the pump is moving. Now, for the other half of the cycle, as the diaphragm switches from the pull (intake) stroke, to the push (exhaust) stroke, the diaphragm now pushes on the fuel in the body of the pump. The flap on the inlet side of the pump, is now being pushed against the inlet port, closing the inlet port. The flap on the outlet side is now pushed open by the the fuel the diaphragm is now pushing against, and the fuel is pushed out of the pump up the the fuel filter.

I think where the "picture in your head" is getting fuzzy is what a check valve is. An actual check valve (like the one that Wes is selling) is a device that has a positive sealing design to it. There is a ball and spring that is calibrated to a certain pressure (even if it .5 or 1 lb), the spring pushes the ball closed causing a seal. No flow happens until the spring seat pressure is overcome. Once you have sufficient pressure to push the ball against the spring and move it out of the way, there is no flow. The reed valve flaps (which are some kind of "rubber" in the lift pump) are just that, a flap. There is no return spring to push them shut (other than the weight of the fuel in the lines and the pump body). So a new pump with healthy flaps might act as a check valve. But as the flaps age and get more pliable as they age, the fuel can sneak past them, since there is nothing pushing them closed like true check valve.

Think of it like this. If a semi is moving forward down the road, the pressure (wind moving past the truck in one direction (the diaphragm pump moving in the lift pump scenario)) will push the rubber mud flap away from the tires in front of it. But, if the truck is not moving, the breeze blowing (now the wind is moving, not the truck (diaphragm pump not moving in the lift pump scenario)) the breeze (fuel) can move the mud flap where ever it wants to. It can push it away from the tire, or bang it into the tire, since there isn't the wind moving past the mud flap to keep in in one position.....

I think that this is what Seladoor was trying to get at with his comment of "like an air compressor". Once he said that I was also thinking about reed valves. I like two stroke dirt bikes so I understand reed valves.

 

[PROFG]

Thanks snicklas, xlnt xplanation, I don't have to tear my almost working LP apart now. I do miss when you could buy a fuel pump kit with diaphragm, 2 valves, and a gasket for couple bucks tho.

 

[dgr]

So it's either a piston port or reed valve two stroke fuel pump. Anyone else excited about the findings?

Here's an Oldsmobile fuel pump

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Tim Ferguson posted some cool info in the comments on how a mechanical fuel pump regulates pressure at all rpms.

 

[PROFG]

Ok, getting a handle on this system I think. Since I have a OEM style LP with minimum, measured, flow and still working on good epump, plan to install early LP leakage detection and flow increase system. Details with pics AFTER installation/testing.

 

[Selahdoor]

Like I said before...

Reed valve. Check valve. Same difference.

It "checks" the flow to let it go only one direction.

If the valve is designed to leak a bit, or it is simply leaking because it is weak or bad... No difference. It is supposed to let the fuel flow only one direction.

No matter what technicality you quote, or nit you want to pick, it is a check valve. And without it our diaphragm pumps will not work.

 

[PROFG]

I think will try external check valves to see if flow improves from weak LP. Pics to follow.

 

[Thewespaul]

Whatever floats your boat they still don’t keep fuel from draining back either way...

Like I said before...

Reed valve. Check valve. Same difference.

It "checks" the flow to let it go only one direction.

If the valve is designed to leak a bit, or it is simply leaking because it is weak or bad... No difference. It is supposed to let the fuel flow only one direction.

No matter what technicality you quote, or nit you want to pick, it is a check valve. And without it our diaphragm pumps will not work.

 

[Selahdoor]

I think will try external check valves to see if flow improves from weak LP. Pics to follow.

Good idea!

Whatever floats your boat they still don’t keep fuel from draining back either way...

Ah. There you go. Now, THERE you have a point.

And I guess that's about where we got started with this, isn't it? Wondering why it would allow leakage backward.

I guess it doesn't matter what you call it. Nor does it make any difference whether it SHOULD prevent backflow or not.

It does.

So we need to deal with that fact with at least one external check valve between the FP and the filter housing...

 

[Selahdoor]

It's not just my opinion, or what 'floats my boat':

Reed valves are a type of check valve which restrict the flow of fluids to a single direction, opening and closing under changing pressure on each face. Modern versions often consist of flexible metal or composite materials (fiberglass or carbon fiber).

https://en.wikipedia.org/wiki/Reed_valve


Looking back, gotta say I'm sorry I got sucked down that rabbit hole.

Oh well. At least as usual, we learned a few things.

 

[Thewespaul]

Yep, agreed there. I guess my mind deals more with the physical, what I have actual experienced and seen with my own eyes happen, versus the technical, terminology and how things should work through principle. No hurt feelings, it’s just an exchange of ideas. It’s good to talk about these things so we can work out how to improve on the parts of our fuel system that is problematic.

Personally, I like to put my check valves at the lowest point of the fuel system, at the rubber hose between the frame and fp. It keeps fuel primed in the entire fuel system around the engine, and as long as you have no external leaks it keeps the engine starting right up every time.