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'89 R3500 Crew Cab 2wd to 4wd conversion & beyond

Started out with 2wd TBI350 with SM465 to current 4wd with 454, 700r4, NP241

skunked

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That's just weird. If it bends easily I'm wondering if fuel slosh could be a legitimate possibility.
 

imiceman44

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Well, well, well, if any of you follow @the_blaze's Plan B build thread, this will look quite familiar.

View attachment 393416

A bent sending unit! It looks exactly like the one he pulled from his tank. I definitely did not cave in the bottom of my tank. The drive shaft would be living in the tank if I did.

View attachment 393423

Here's the depth of the tank. It's measured from the long end of the square that starts at 24".

View attachment 393417

View attachment 393424

So it's basically 11-7/8". The bent up sender measures well short of the bottom and about 10-1/4".

View attachment 393418

I can't see any way the tank was sucked or dented in 1-5/8" without any outward indication. I can push down and get minimal movement on the top of the tank, certainly no where near 1-5/8".

I pried on the baffle tray with the square and it's solidly afixed.

View attachment 393419

You can see a mark where the bottom of the sender scraped on the tank.

I could easily bend the tube back to straight and the sender sits down into the tank easily as referenced in the picture below:

View attachment 393420

I could move the locking ring around 3/4 of the way to fully seated by hand. That confirms there is plenty of room between the sender and the bottom of the tank.

The only working theory we've come up with is the tube is so weak it's getting bent by fuel slosh.
I have a new theory now and it is probably what happened to both of you.
How air tight is your tank?
When you open the cap when cold does it feel like it was vacuum?
That could pull the flat part in and cave in easily one or two inches and spring back.
Probably little bit on top and little on the bottom.
 

skunked

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I have a new theory now and it is probably what happened to both of you.
How air tight is your tank?
When you open the cap when cold does it feel like it was vacuum?
That could pull the flat part in and cave in easily one or two inches and spring back.
Probably little bit on top and little on the bottom.

I thought about that however that's the reverse of what I've experienced on my trucks. Usually it's positive pressure......
 

imiceman44

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I thought about that however that's the reverse of what I've experienced on my trucks. Usually it's positive pressure......
We live in warm weather.
They live in Utah and Colorado.
I believe they already have snow
 

imiceman44

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I thought about that however that's the reverse of what I've experienced on my trucks. Usually it's positive pressure......
Notice that the crush is identical, they got compressed to the same length.
I don't know what else could do that with no evidence
 

mrk5

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Every time I've removed the cap there's pressure in the tank, or the flip side of that I've never had a vacuum in there that I've observed. I do have a vented cap, although I don't recall now if those caps vent pressure or relieve vacuum. The vent line is open ended because I removed the charcoal canister some time ago and never replaced it; aside from that it's just the factory plumbing. I can certainly check the line to make sure it's clear.

I have no idea what kind of metal gauge the tank is. I doesn't seem too flimsy. Barring an obstruction in the vent line, I'm not sure what I could do to combat a collapsing tank.

Barring a collapsing tank, it's either gotta be fuel slosh or I bent it on install. Either of those causes indicates the tube is extremely easy to bend because I know I didn't force that thing into place.

My plan is to work with my current sender. I could run a brace from up at the top down to the bottom of the bent line to help hold it in place.

Or I have another idea if it's the tank collapsing that caused this. I could use a different pump with a threaded fitting on the bottom, shorten up the sender tubes, and use a hose on the suction of the pump going down to a HydraMat pickup. The hose would allow the tank to collapse without permanently damaging anything.
 

folkenheath

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So to me it seems like the big scratch in the bottom of the tank is a clear sign that the bottom of the tank hit the sending unit and bent it up.

Is the vent tube open in the sending unit? See if it flows air? It shouldn't build pressure(or vacuum) if you have an open vent, something is blocking the vent. And if it wasn't hit from the bottom, it's possible with the vent being plugged as the pump sucks fuel out, and air can't come in, it's collapsing the tank...

I'm thinking the vent tube in the sending unit isn't actually venting like its supposed to, or the fuel cap, or both. You are getting a vacuum, that pump can make a lot of pressure, and if the tank can't let air in to displace the volume of the fuel that was removed, something has to give, it doesn't take much on that much surface area. If the top of the tank is 30x30 for example, even 1 psi of vacuum is 900 lbs of force on the top and bottom of the tank.

Get the vent working properly, and your issue will likely go away. Don't brace it, or you will put all that force onto the one point of the brace instead of spread out and likely kink or bend something permanently.
 
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imiceman44

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So to me it seems like the big scratch in the bottom of the tank is a clear sign that the bottom of the tank hit the sending unit and bent it up.

Is the vent tube open in the sending unit? See if it flows air? It shouldn't build pressure(or vacuum) if you have an open vent, something is blocking the vent. And if it wasn't hit from the bottom, it's possible with the vent being plugged as the pump sucks fuel out, and air can't come in, it's collapsing the tank...

I'm thinking the vent tube in the sending unit isn't actually venting like its supposed to, or the fuel cap, or both. You are getting a vacuum, that pump can make a lot of pressure, and if the tank can't let air in to displace the volume of the fuel that was removed, something has to give, it doesn't take much on that much surface area. If the top of the tank is 30x30 for example, even 1 psi of vacuum is 900 lbs of force on the top and bottom of the tank.

Get the vent working properly, and your issue will likely go away. Don't brace it, or you will put all that force onto the one point of the brace instead of spread out and likely kink or bend something permanently.
I didn't think about the pump creating the vacuum but it makes more pressure than just extreme cooling.
I have a feeling we solved the mystery pending confirmation that the vent is not working properly
 

mrk5

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The vent line isn't plugged. I can blow 60psi air threw it. It's pretty difficult, but I can blow into it. I certainly wouldn't say it flow easily.

I did notice these interesting dimples on the top of the tank.

2021-10-20 14.24.45.jpg

2021-10-20 15.31.59.jpg

2021-10-19 14.40.31.jpg

I scrolled thru pictures during fabrication and I can't see the dimples in those pictures.
 

folkenheath

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The vent line isn't plugged. I can blow 60psi air threw it. It's pretty difficult, but I can blow into it. I certainly wouldn't say it flow easily.

I did notice these interesting dimples on the top of the tank.

I scrolled thru pictures during fabrication and I can't see the dimples in those pictures.
Which way? Can air enter the vent line? Or is that the caps job? I'm not sure which is supposed to let the air in, but I feel like whichever it is, it's surely not happening, especially seeing those dimples there from the top likely bowing in too, to the point it kinked the sheetmetal. I know with an old mechanical pump, where it only sucks the fuel, a blocked vent can actually prevent the fuel from being drawn out, or drawn out fast enough to make decent power.

However, with internal electric pumps that simply force the fuel out, it could create a much stronger vacuum from inside if the air can't get in.

EDIT: I just reread it, you can blow 60 psi air in? How much pressure does it take before air can enter the tank?
 
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mrk5

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I only blew the air in the "out" direction from the tank to the engine bay. I didn't try blowing it the other way. Trying to blow thru the line with my mouth was difficult enough it hurt my cheeks.

I do have what should be a vented fuel cap. But like you, I'm not sure if they only work in 1 direction and which direction that is.

There is this contraption in the vent line that I'm not sure what purpose it serves.

2021-10-21 08.53.28.jpg

2021-10-21 08.53.28-1.jpg

I'm guessing that would prevent fuel from being sucked into the evap canister via the vent line? Like it senses a vacuum upstream and closes the valve? Maybe I need to eliminate that?

I'm tempted to run the vent from the sending unit just up in the bed with a rollover valve. My hesitation is fumes. If I have it in the garage, I don't want it to smell up the garage or shop.
 

Cheavyk10

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Vented gas caps work in one direction, to release pressure (out). The center of your cap on the inside should have a spring loaded "button". You should be able to push it in with a screwdriver. I found that mine was to stiff to release the pressure in my tank. I drilled a hole all the way through the cap, which worked to release pressure, but now the cap leaks gas on occasion.
 

sweetk30

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I only ever run a line direct to vent port .

I would ditch that thingy and connect the lines and re-test .

I have never seen one of them on all my square nody trucks i have run or parted out .
 

folkenheath

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What about if you bypass that little vent loop you got there, can you blow air in then?

It wouldn't take much vacuum at all to collapse the tank inward with that large of area.

Also make sure that little bulbous looking thing on the end of the vent line inside the tank is allowing air in. Also, I've almost accidentally hooked up the return line to the vent line. The return line should be the one that goes to the bottom of the tank but has no sock like the pump. That way it doesn't bring air bubbles in with it.

I put a small filter on the end on my vent line.
 

mrk5

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That doo-hickey was on there from the dual tank setup. I tried to do some searching to find out more about it, but can't find anything.

@folkenheath I do know I have the lines correct on the sender. I messed that up with the PF4 install in the C10. For some reason the carb trucks use a 5/16" vent but the TBI trucks have a 1/4" vent line. When I hooked up the lines on the C10 I figured they were the same as TBI. Then I couldn't figure out why fuel pressure wouldn't come down.

Found this interesting read on venting tanks: https://www.onallcylinders.com/2015...f-installing-a-tanks-inc-rollover-vent-valve/

So this is my plan:
  1. Re-use my current sender with a brace for the bent line. Even if I don't have the same problem again, it's too easy to bend that thing.
    1. Double check the vent line in the sender is clear.
  2. Replumb the vent line to a rollover valve on the bed. Since I have already demonstrated the ability to lay the truck over, a rollover valve is a good idea.
I'm debating about using some of our structural glue to chemically weld a piece of angle across the width of the tank. Just wondering if now it might be compromised.
 

mrk5

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More photos of the doo-dad below. It's definitely restricting air flow. As soon as I pull the hose off pressure was bleeding off the tank. I can barely blow thru it from the tank side and I can't blow thru it at all from the engine bay side. Which means if there was a vacuum in the tank, it wouldn't get relieved thru the vent line.

2021-10-21 14.46.57.jpg

2021-10-21 14.46.37.jpg

2021-10-21 14.46.10.jpg

2021-10-21 14.47.02.jpg
 

imiceman44

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I'm thinking some of the problem may be not having the evap canister installed. That watchya-whoseit might only flow with a vacuum on the engine side being drawn thru the evap canister.
So you found your culprit.
 

mrk5

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So you found your culprit.
So it would seem. Might have been fine if I had the evap canister installed and properly connected. I just hate having to deal with all that vacuum plumbing if I don't have to.

I still haven't found anything about that device being used in the fuel vent, but I haven't poured thru my service manuals yet either. The only thing that comes up is it being used as a purge valve on the evap canister.
 

folkenheath

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If your test shows you can blow air into the tank without it, I would just eliminate it and install the rollover valve you mentioned.

It's possible its defective and won't work even with the engine vacuum, a test with a hand vacuum pump would tell you that. I guess it would tell you if that worked, if it didn't you wouldn't know if it was defective or wasn't supposed to work that way.
 
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