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Air in fuel line - what to do next?

rumblecloud

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I'm goin' with RonD and your assumption of where you think the air is entering, starting with the in tank pump:

- 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
Seems too obvious, but they gotta be starting somewhere before you see them in the translucent line.
 


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If there was a leak anywhere in the lines/high pressure fuel pump after the in tank sending unit... there would certainly be visible signs of fuel leakage.

The only reasonable explanation I can think of is that there is a crack in the sending unit fuel delivery tube between the in tank pump and where it meets the top of the sending unit... but internal to the tank. anything after that would have visible signs of fuel leakage.

Maybe try taking the lock ring off the tank and pulling up the sending unit a bit... while keeping the low pressure pump in the fuel... cycle the pump on a few times and look for a leak where the output tube is connected to the top of the sending unit.
 

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If there was a leak anywhere in the lines/high pressure fuel pump after the in tank sending unit... there would certainly be visible signs of fuel leakage.

The only reasonable explanation I can think of is that there is a crack in the sending unit fuel delivery tube between the in tank pump and where it meets the top of the sending unit... but internal to the tank. anything after that would have visible signs of fuel leakage.

Maybe try taking the lock ring off the tank and pulling up the sending unit a bit... while keeping the low pressure pump in the fuel... cycle the pump on a few times and look for a leak where the output tube is connected to the top of the sending unit.
Most local parts stores have a metal repair line . It will allow you to hook into the line at the tank and hook up the other pump with a rubber gas line that you originally removed , and run on a gas can. If it still gets air , Unhook the feed line on the the canister to the high pressure pump move the make shift set-up to that line and restart. This should pin point where the problem lies. Hope this helps.
 

fixessaxes

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Yeah, similar thoughts here now. It was a delphi part and afaik that is a reputable brand, but I suppose I could have gotten unlucky. Also getting to the end of my abilities, so after I pull it back out if I can't figure anything there it may just be getting towed in shame to my mechanic.
 

RonD

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So you replaced the whole assembly?
If so then it would be very unlikely(unlucky) to have the same air issue on new assembly
Did you change or clean the "sock" filter?

A pump can have/cause cavitation, like on a completely submerged boat propeller, air bubbles are generated by spinning blades, cavitation

That can also be caused by limiting flow IN to pump, i.e. the "sock", or limiting flow OUT, restrictions

Good read here on fuel system cavitation: https://www.fuelab.com/blog/what-is-cavitation-how-to-avoid-it/76

Gasoline has a fairly low vapor point at the low pressure pump end so will easily form bubbles if inlet is restricted
 

fixessaxes

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Interesting. The new assembly came with a new sock, but I could try taking it off just to see what happens.
 

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You had an issue with the original pump that was separate from this one, correct - i.e. the aeration problem is brand new and came with the Delta pump?
 

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Actually, any air would be routed to the return side by the fuel vapor separator on the left frame rail that is its job also some of those had a fuel filter in them, my 86 does but be careful because at some point Ford eliminated that filter and basically glued the reservoir on and you could break it trying to remove it and good luck trying to find a replacement. By the way most times when the rear pump is bad it will run fine on a full tank but when it drops to about 1/2 tank it will run poorly, I personally had this happen to me on my Ranger. I made it into a gas station and filled it up and it ran fine. Why does your truck have a clear line on it? Most likely not fuel rated material consider replacing it with the correct style line.
 

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I also think a fuel pressure test would be the next step I would take. You should be able to rent a kit at Autozone, the deal is you pay for the tool when you rent it and when you return it you get a full refund. Note the fuel pressure with it running and see how fast it falls off when you shut it off and also check it key on engine not running.
 

fixessaxes

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What should the fuel pressure look like if I rent that tool? This is the bike valve next to the head, right?

Yes, the issue I was having before was much less intense than this. I didn't check if there was air in the line before- the truck drove just lost power under load when the tank was low, leading to my assumption I had a low pressure pump that was going bad.

So since posting last I have swapped the new low pressure pump with the old low pressure pump and... same problem. Air in the line, won't run for more than 10 seconds or so, less if it hasn't been sitting, tons of air in the line. Again: this is not the problem it had previously. See paragraph above. I know its confusing and annoying believe me; I'm living it.

Next thing I did was swap out the fuel line going from the tank to the accumulator/reservoir and.... same problem. So guesses are: air is either entering the fuel system at the reservoir or I have an electrical problem preventing the low pressure pump from engaging once the truck is running (whose source is not the fuel pump relay because I already changed that out).


Going to have a mechanic friend come look at it on Saturday, last stop before the tow truck of shame.
 
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89Twincharge

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Pull pump/sending out and check rubber hose on lift pump... Never use the hose they give you with Delphi pumps they melt... Buy ethenal rated hose and use metal hose clamps(instead of plastic they come with sometimes) had this happen 4 or 5 times at the shop and after the last comeback I just used e-rated hose and haven't had a comeback since..
 

fixessaxes

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Nice tip, thank you.

New possibility I have learned about is a check valve stuck closed in the reservoir. I can bypass that reservoir with some tubing and a check valve, might do that.
 
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fixessaxes

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FIXED.

Bypassing the reservoir/accumulator did the trick. I am guessing the check valve in the reservoir was stuck mostly shut, causing air bubbles when the high pressure pump pulled a vacuum. It would allow a trickle past, which is why I could keep starting it. The bowl of the reservoir was not leaking, so there was no actual air ingress, just gas boiling under negative pressure.

An unusual and rare problem, I think. Now I just need to figure on whether I want to permanently bypass the reservoir and if so how, or whether I want to try to fix or replace it. I am under the impression that my year has a reservoir that is not designed to be serviceable, and that the check valve is way up in there even if you do get it open.

Thank you everybody who responded here, I learned an awful lot and probably would have had it towed earlier (and likely paid a lot to have this weird problem tracked down) if it weren't for the suggestions and feedback here.
 

Jazzer

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My ‘87’s was glued shut. Might want to source a junkyard one.
Nice job getting it running.

-Jazzer
 

fixessaxes

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Thank you! Did you ever have any issues with yours being bypassed?
 

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