Fuel system drawing a vacuum

[Ferroequinologist]

Ok, I'm pulling my hair out. Just did tons of work to correct this issue, yet it still occurs

I can let the truck idle and it dies after about 5 min. I can drive it and it dies after .25 miles. Open the bleeder screw on the replacement filter housing, or open the filter drain, and it sucks in air like it is under a vacuum. Take the fuel pump line loose from filter housing, crank engine, and the fuel will shoot out over the passenger fender...

-Brand new tank
-Brand new pickup tube and sock
-Fresh known clean fuel
-Hard lines blown out first each way with air
All new rubber lines, injector hard rubber not likely to collapse
-changed filter a few times

I'm at a loss...

 

[ezgn]

Ok, I'm pulling my hair out. Just did tons of work to correct this issue, yet it still occurs

I can let the truck idle and it dies after about 5 min. I can drive it and it dies after .25 miles. Open the bleeder screw on the replacement filter housing, or open the filter drain, and it sucks in air like it is under a vacuum. Take the fuel pump line loose from filter housing, crank engine, and the fuel will shoot out over the passenger fender...

-Brand new tank
-Brand new pickup tube and sock
-Fresh known clean fuel
-Hard lines blown out first each way with air
All new rubber lines, injector hard rubber not likely to collapse
-changed filter a few times

I'm at a loss...

Have you cracked the injector lines and bled all the air out at the injectors? You said it squirts fuel out of the fuel pump, is it a new pump, and the right pump? It might be an Injection pump issue. You probably already know, that there are two fuel pumps that fit but only one is the correct one that works. I don't know the history of the truck. Just saying. If you are getting fuel at all the injectors without any hard start issues, then I would suspect Your IP or your injectors. A fuel pressure test could help diagnose the problem.

 

[Ferroequinologist]

Have you cracked the injector lines and bled all the air out at the injectors? You said it squirts fuel out of the fuel pump, is it a new pump, and the right pump? It might be an Injection pump issue. You probably already know, that there are two fuel pumps that fit but only one is the correct one that works. I don't know the history of the truck. Just saying.

If I open the filter bleed and the pressure equilizes, and crack fuel lines, it will fire back up and run the same. Fuel pump doesn't have many miles on it but was replaced the last time I was trying to solve this issue. I didn't know there were two pumps that fit, but I was pretty sure I had looked up the part number here when I replaced it.

I have a clear tube on the injection pump return and there will be some bubbles right after I get it reprimed and running, but they go away several minutes before it dies again.

 

[ezgn]

If I open the filter bleed and the pressure equilizes, and crack fuel lines, it will fire back up and run the same. Fuel pump doesn't have many miles on it but was replaced the last time I was trying to solve this issue. I didn't know there were two pumps that fit, but I was pretty sure I had looked up the part number here when I replaced it.

I have a clear tube on the injection pump return and there will be some bubbles right after I get it reprimed and running, but they go away several minutes before it dies again.

Delphi Hfp906

 

[Ferroequinologist]

I'm at the store getting a new lift pump now. Only thing other than the IP

Yep that one.

 

[ezgn]

I'm at the store getting a new lift pump now. Only thing other than the IP

Yep that one.

If I remember correctly the correct fuel pump if you pull down on the lever it will immediately have resistance and begin to pump. on the incorrect one, the lever will have some play before it engages.

 

[Ferroequinologist]

If I remember correctly the correct fuel pump if you pull down on the lever it will immediately have resistance and begin to pump. on the incorrect one, the lever will have some play before it engages.

Ah ok thanks.

 

[nyoffroad]

Ok, I'm pulling my hair out. Just did tons of work to correct this issue, yet it still occurs

I can let the truck idle and it dies after about 5 min. I can drive it and it dies after .25 miles. Open the bleeder screw on the replacement filter housing, or open the filter drain, and it sucks in air like it is under a vacuum.

I'm at a loss...

Maybe it is a vacuum, have you tried loosening the filler cap and see if it will continue to run?

 

[ezgn]

Maybe it is a vacuum, have you tried loosening the filler cap and see if it will continue to run?

Maybe the ball check valve at the lift pump. Maybe a clogged fuel tank vent line.

 

[Mullaney]

If I open the filter bleed and the pressure equilizes, and crack fuel lines, it will fire back up and run the same. Fuel pump doesn't have many miles on it but was replaced the last time I was trying to solve this issue. I didn't know there were two pumps that fit, but I was pretty sure I had looked up the part number here when I replaced it.

I have a clear tube on the injection pump return and there will be some bubbles right after I get it reprimed and running, but they go away several minutes before it dies again.

.
Adam,

I don't have one and the one I did have was gas and not diesel... On my gas version, there was a black plastic "bottle" down low on the passenger side of the truck. It was for fuel vapor control (sorry, I can't remember the real name for it). It had a pair of hoses that ran to the fuel tank and back. Can't really remember where it plugs into the motor somewhere (vacuum I think). Could be possible that something there is stopped up. Especially since it will run for a bit, then quit - reliably.

@nyoffroad 's suggestion about loosening the filler cap is a good way to prove that idea too. If the truck is parked - heck - just remove it for a few minutes and see if that solves the problem.

 

[Ferroequinologist]

Maybe it is a vacuum, have you tried loosening the filler cap and see if it will continue to run?

I have tried it with the cap loose and off completely, same problem occurs.

Got a replacement lift pump on order, no one had the right one in stock locally.

 

[Keith_J]

Sounds like partial vacuum in the tank. The lift pump provides a pressure of around 5 PSI gauge referenced to atmospheric pressure by spring force on a Viton-fabric diaphragm about 3 inches in diameter. That is an area of 7 square inches, the spring force is about 35 pounds. There is also 14.7 PSIA on the spring side, a force of about 100 pounds, from the column of air we live under.

It doesn't take much vacuum in the fuel tank to completely over power the lift pump. Yes, there is the vane pump in the injection pump but it too has a limit of suction, once reached it becomes a heater and cavitates.

Why does this happen? Fuel tank caps have vacuum relief valves which can fail. Caps must seal and not leak in a roll over accident plus must retain specific pressure. All of these functions are typically sealed with fuel resistant rubber like Viton which can age and become adhesive to the valve seat.

These fuel caps are hen's teeth..rare and hard to find. I've yet to tear into one because of this. If you tear into it, don't damage the threads. I believe the body is glass filled Nylon (PA6 GF) so it can be fixed with epoxy adhesives.

I would pull the filler neck and vacuum test. I bet it could be "fixed" with correct application of vacuum. The fuel tank won't take much vacuum but the filler neck is good for full vacuum. You will need a good plug for the hose end.

 

[Ferroequinologist]

Sounds like partial vacuum in the tank. The lift pump provides a pressure of around 5 PSI gauge referenced to atmospheric pressure by spring force on a Viton-fabric diaphragm about 3 inches in diameter. That is an area of 7 square inches, the spring force is about 35 pounds. There is also 14.7 PSIA on the spring side, a force of about 100 pounds, from the column of air we live under.

It doesn't take much vacuum in the fuel tank to completely over power the lift pump. Yes, there is the vane pump in the injection pump but it too has a limit of suction, once reached it becomes a heater and cavitates.

Why does this happen? Fuel tank caps have vacuum relief valves which can fail. Caps must seal and not leak in a roll over accident plus must retain specific pressure. All of these functions are typically sealed with fuel resistant rubber like Viton which can age and become adhesive to the valve seat.

These fuel caps are hen's teeth..rare and hard to find. I've yet to tear into one because of this. If you tear into it, don't damage the threads. I believe the body is glass filled Nylon (PA6 GF) so it can be fixed with epoxy adhesives.

I would pull the filler neck and vacuum test. I bet it could be "fixed" with correct application of vacuum. The fuel tank won't take much vacuum but the filler neck is good for full vacuum. You will need a good plug for the hose end.

I have taken the cap completely off, and it still does the same thing. Filler neck is completely open to the tank and atmosphere. The tank is not getting a vacuum on it. I did use my AC vacuum pump, a piece of plexiglass, and a hose and the cap vent does vent under vaccum.

I know how hard the caps are to find. I lost the first one and it took 2 years to find someone willing to sell me a spare they had.

 

[nyoffroad]

I have taken the cap completely off, and it still does the same thing. Filler neck is completely open to the tank and atmosphere. The tank is not getting a vacuum on it. I did use my AC vacuum pump, a piece of plexiglass, and a hose and the cap vent does vent under vaccum.

I know how hard the caps are to find. I lost the first one and it took 2 years to find someone willing to sell me a spare they had.

OK, next thing I'd be looking at is the fuel lines, in the OP you said you changed the tank. Perhaps you pinched a line between the tank and frame?

 

[Ferroequinologist]

OK, next thing I'd be looking at is the fuel lines, in the OP you said you changed the tank. Perhaps you pinched a line between the tank and frame?

Good thought, I just double checked them. They look in the clear. I was having this issue before replacing the tank and lines. I (rightly so) assumed the tank was gunked up and the sock was plugged.

 

[Keith_J]

Fuel hose has at least three layers, these can delaminate without outward appearance of damage. The inner, most fuel resistant layer can collapse, causing such symptoms as seen.
Fatty acid methyl ester (FAME, euphemistically called biodiesel) will attack aging rubber, causing this problem. Most pump diesel contains FAME to replace lubricity lost with removal of sulfur compounds to meet ultra low sulfur diesel.

 

[Ferroequinologist]

OK, next thing I'd be looking at is the fuel lines, in the OP you said you changed the tank. Perhaps you pinched a line between the tank and frame?

I did check that and they are all free.

 

[Ferroequinologist]

Fuel hose has at least three layers, these can delaminate without outward appearance of damage. The inner, most fuel resistant layer can collapse, causing such symptoms as seen.
Fatty acid methyl ester (FAME, euphemistically called biodiesel) will attack aging rubber, causing this problem. Most pump diesel contains FAME to replace lubricity lost with removal of sulfur compounds to meet ultra low sulfur diesel.

Right, which is why I changed every hose with brand new.

 

[Mullaney]

Right, which is why I changed every hose with brand new.

.
So... Back to zero and the least expensive and simplest way to try something new. What if you had a fuel can and some brand new hose out to the truck and a second hose back to the fuel can?

It only needs to run for 10 of 15 minutes to be better than what you have now if I remember correctly?

 

[Squibbly]

I've never tried it but can't you strap a small gas tank on top of the batteries and run some clear fuel line with a boat squeeze bulb (primer bulb - walmart) in it (or electric pump from a tank on the ground) to prime the line from it to the line "after" the filter or directly to the IP? That will cut off all the possible reasons from the filter back for troubleshooting purposes, no?

I'd try to eliminate as many systems and lines as possible pre-IP, then work my way back until the issue occurs if it isn't the IP itself.

(Any line you disconnect post lift-pump will need to go into the portable gas can.)