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Air in fuel line - what to do next?

fixessaxes

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1987 Ranger, 2wd, 2.9. Starts and then dies 30 seconds later. I can see bubbles in the fuel line that start a few seconds after startup, increasing until its basically just air and not much fuel and then the engine dies.

Originally, I was losing power when the tank was less than half full, so I figured it was time to change out the fuel pumps. Pulled off the bed and changed out the low pressure pump in the tank, replaced the high pressure pump under the drivers side door, changed out the fuel filter as well. Inside of gas tank was clean. Truck starts fine, but starts pulling air into the fuel line shortly after start.

In this truck, the series goes:
gas tank -> low pressure pump -> line -> fuel accumulator/reservoir -> line -> high pressure pump -> line -> filter -> onwards to engine

Between the fuel accumulator and the high pressure pump, the line is translucent so I can shine a line and see the air bubbles start up shortly after startup. The line between the low pressure pump and the accumulator is opaque, so I can't see in there. I CAN detach the line from the low pressure pump and turn the key and see that the pump is working when it spurts gas out.

Other weird detail is it isn't leaking gas anywhere. Just sucking in air somewhere between the low pressure pump and the high pressure pump. The places I can think that could be it are:
- junction between line and low pressure pump
- line between low pressure pump and accumulator
- junction between line and accumulator
- accumulator itself
- junction between accumulator and line going to high pressure pump

I can see bubbles throughout the entire length of the line from accumulator to high pressure pump, so I don't think the leak is in that line.

Again, no gas leaks anywhere. Just air getting pulled in after startup. I am now thinking that the pumps were fine (although its nice to have a working fuel gauge now, so no regrets on the low pressure pump changeout at least) and that I had a small air leak that I made worse through fussing with the lines while working.

I don't imagine that there is some sort of problem with the (new) low pressure pump- if that was somehow not strong enough or stopped working after startup (remember I can detach the line and see when I turn the key that it shoots gas out) I should just lose power, not introduce air into the lines, right?

This has been a pretty annoying problem to get this far, so I figure I would ask for some wisdom or advice before I go any further. Any thoughts from anyone? Anything I am overlooking? Any easy tests I can do to narrow it down? What should I do next? I am not super experienced at working on cars, but work with my hands for a living. So I am somewhat handy, just not experienced.


EDIT FOR ANYONE WHO GETS HERE FROM GOOGLE IN THE DISTANT FUTURE:
It ended up being a broken check valve inside the fuel accumulator/reservoir. It was sealed mostly shut, allowing some fuel by so the truck could start, but then the high pressure pump would pull a bit of vacuum and the gas would "boil" in the low pressure, showing up as the air bubbles I saw in the line forward of the accumulator. As for the check valve inside the accumulator, it is not a user-serviceable part, and a rare problem. It must have been breaking slowly, and when I put new pumps on I jostled it or the new pumps were stronger or something and busted it the rest of the way. Anyhow, I took the fuel accumulator off, I am going to remove the check valve entirely, and then put it back together with a new O-ring and install an inline check valve just before the accumulator. Everything will still operate as intended, and it will be a cheap fix.
 
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ericbphoto

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Check to make sure the low pressure pump is working. If it’s failed, the high pressure pump is trying to suck fuel through it. So that might be causing enough vacuum (suction) to cause fuel vapor bubbles to form. Also make sure filter is clean. There is usually also a filter sock on the bottom of the low pressure pump in the tank. If that is really dirty, that would be another restriction to flow.
 

fixessaxes

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Sock is clean, tank is clean. Fuel filter is new and also is after the air bubbles, past high pressure pump. For the low pressure pump, I can detach the line and see when I turn the key that it shoots gas out, so it least works at startup. Is there a better way to test it?
 

rusty ol ranger

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Hows the fittings ontop of the tank?
 

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It reads like the hose from lift pump(in the tank) to top of tank has a leak
Reason is no smell of fuel reported, so no external leak
And mention of the below 1/2 tank thing, once hose in tank was exposed to the air it could suck it in
 

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It reads like the hose from lift pump(in the tank) to top of tank has a leak
Reason is no smell of fuel reported, so no external leak
And mention of the below 1/2 tank thing, once hose in tank was exposed to the air it could suck it in
Agreed
 

fixessaxes

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Hows the fittings ontop of the tank?
They feel loose to me, but I have nothing to compare it to. I took the outlet line off, put vaseline on the metal part, and reinstalled it to see if it might temporarily seal it and halt the problem but no difference. Not sure if that is a good enough test. Can I replace the little fittings? Just cut old one off, put new one in the existing line? Where can I find those?


Fuel tank pump is new replacement. Tank is currently full.
 
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rusty ol ranger

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They feel loose to me, but I have nothing to compare it to. I took the outlet line off, put vaseline on the metal part, and reinstalled it to see if it might temporarily seal it and halt the problem but no difference. Not sure if that is a good enough test. Can I replace the little fittings? Just cut old one off, put new one in the existing line? Where can I find those?


Fuel tank pump is new replacement. Tank is currently full.
They are spring lock so a bit of looseness is normal.

I think @RonD is onto something, he usually is
 

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since its full, you are still getting bubbles and its dying?

are you sure its not your fuel tank vent is clogged so your pump is just creating a vacuum until it cannot pull anymore fuel?

try running the truck with the gas cap off and see if still does the same thing. if it does, then its not the tank vent but if your truck doesn't die than you know its a vacuum issue.
 

fixessaxes

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Yes, tank is full. And the low pressure pump was just replaced. Gas cap is off because bed is still off.

The air bubble in the line are pretty extreme- it goes from normal looking to aerated like a fish tank to mostly air in about 30 seconds.
 

rusty ol ranger

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Yes, tank is full. And the low pressure pump was just replaced. Gas cap is off because bed is still off.

The air bubble in the line are pretty extreme- it goes from normal looking to aerated like a fish tank to mostly air in about 30 seconds.
If its sucking that much air somewhere youd think itd be obvious
 

fixessaxes

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Yeah. I even put on my Peltors to see if I could hear it when the engine was running if everything was attenuated. Pretty frustrating & dumb problem with what I am sure is an obvious solution.

Could it just be that the system is so full of air and a normal prime-prime-prime isn't enough? I had my someone prime it a bunch while I watched the fuel line (the translucent part between the accumulator and the high pressure pump where I can see bubbles if I hold a light behind it) and it pulled air through, then the air went backwards a bit after the pump stopped.
 
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rusty ol ranger

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Yeah. I even put on my Peltors to see if I could hear it when the engine was running if everything was attenuated. Pretty frustrating & dumb problem with what I am sure is an obvious solution.

Could it just be that the system is so full of air and a normal prime-prime-prime isn't enough? I had my someone prime it a bunch while I watched the fuel line (the translucent part between the accumulator and the high pressure pump where I can see bubbles if I hold a light behind it) and it pulled air through, then the air went backwards a bit after the pump stopped.
Any air trapped should get pushed into the injectors and into the cylinders pretty fast.
 

fixessaxes

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I can report it is also not the fuel pump relay. It was pointed out to me elsewhere that the relay is bypassed during startup, so if it was bad, I would experiencing symptoms similar to what I am having. So I replaced that relay, with much hope in my heart, and no difference.
 

rusty ol ranger

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I can report it is also not the fuel pump relay. It was pointed out to me elsewhere that the relay is bypassed during startup, so if it was bad, I would experiencing symptoms similar to what I am having. So I replaced that relay, with much hope in my heart, and no difference.
Im wondering if something is defective in the lift pump
 

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