Fixing the Truck to Death – time to cut my losses?

[Drunkcanuk]

I remember how much of a pain those new straps are!!!! Thanks for the PTSD!!!! Lol

 

[PlatonicSolid]

Just checked the dead head pressure and it measures 16psi with air in the line and 19psi when I purge the air out.

19 psi is 19 psi whether it's fuel, air, jello, poop. So it compressed, just means it took a second longer, and reduced volume till it became equal to applied force( pump output ).

Air is compressible, whereas fuel is not. When air is present in the fuel line, it can be compressed by the fuel pump, which can cause a decrease in fuel pressure.

Had to respond to this as it's not a: what weighs more a pound of feathers or a pound of rocks, scenario.

 

[GoToGuy]

Re read the answer I gave. What's gravity have to do with it.
The parameter is a " testing ".

 

[PlatonicSolid]

If you aren't saying that air has no effect on fuel pressure, then I have no idea what that sentence means.

 

[Schurkey]

Air is compressible.

If the fuel pump has adequate volume (NOT just a 2-second "prime") the compressed air DOES NOT change the pressure reading in a static system.

Same deal in a "mechanical" oil pressure gauge with air in the tubing. The air can dampen the movement of the needle due to changes in oil pressure, but in a static system, it has zero effect on the gauge readout.

 

[Schurkey]

what weighs more a pound of feathers or a pound of rocks, scenario.

I posted the old "which weighs more, a pound of iron or a pound of gold" some time ago, maybe on this forum although I didn't find it in a quick search.

There were some interesting but incorrect answers, and eventually correct answers with explanation.

 

[PlatonicSolid]

I'm no physics expert so I can only comment on my real world test experience.
I was testing dead head pressure: Gauge connected directly to pump with no output. Fuel module was not disconnected so I was testing with pump running 20 seconds.
Test 1 - 16 psi
Test 2 - 16 psi (Confirmed result of Test 1)
Test 3 - pressed purge valve on gauge until fuel was in purge line = 19 psi
Test 4 - 19 psi (Confirmed result of Test 3)

I'm not willing to risk pump damage by running it longer, but in above dead head test scenario I suspect the 16 psi test result would remain unchanged regardless of how long the pump is running assuming the pump output force is a constant. If none of the variables of Pressure = Force/Area change, I don't see why pressure would change over time.

It should be noted that there was a significant amount of air in the line for Tests 1 and 2 as most of the fuel had drained out of the line when connecting the gauge.
My quest for info on this topic really takes you down the rabbit hole. Ended up on hydraulic sites detailing the differences between Free Air, Entrained Air and Dissolved Air. My test above qualifies as Free Air (air pocket).

 

[PlatonicSolid]

Progress. I didn’t completely remove the tank. Only dropped it down enough to gain easier access to put the siphon through filler neck hose and remove the old feed and return lines. The new Linestogo fuel lines come with braided stainless flex lines for pump to rigid line connection which is a nice upgrade from OEM crimped rubber hose. The rigid Linestogo lines were formed to an exact match to OEM.

I’m now looking at the TBI flex line replacement, which appears more difficult than expected. The old fuel lines are very securely routed.
Flex lines, in order of support devices, going from under the truck to the TBI:
• Angled bracket attached to frame with 2 securing clips which were easy to remove.
• Custom bracket attached to lower bell housing which appears to be permanently attached to the flex line. Haven’t removed that yet so not 100% on whether it’s a permanent part of the OEM flex line assembly.
• A roughly 7 inch long tube going up the side of the bell housing. No idea how that’s attached or how difficult it’ll be to fish 2 flex lines through it.
• “P” clip attached to top of bell housing. I can see it from under the truck, but gaining tool access to remove and reinstall with new lines looks near impossible.

I’d prefer to follow OEM routing/support, but I’m not sure I can achieve it. I see no guidance in the manuals for flex line replacement.

 

[Scooterwrench]

There were some interesting but incorrect answers, and eventually correct answers with explanation.

Not sure if this would be considered a correct answer but the relationship of fluid vs air is not so much weight as it is density. Air is going to pass by the valve in the FPR easier than fuel and not have to lift it as far off its seat and not compress the spring as much as fuel that does have to push the valve farther off its seat thus compressing the spring more resulting in a higher reading. I know that in the past my testing with air has always given a lower reading than testing the same device with liquid.

 

[PlatonicSolid]

Here are a couple pics of the braided TBI to rigid fuel line routing from under the truck driver side. I see a lengthwise slit in the tube that I assume will allow me to remove old and insert new line without having to actually thread them through. I may just cut the old line and see if I can pull it through that top inaccessible "P" clip. I tried to see if I could access that "P" clip from the engine bay, but I can't see it, nor get a hand down there, nor get a camera shot of it.

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Looks like I can bend the lower clip open to access that one.