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Old 07-04-2019, 12:32 PM   #1
ShortHaul
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'75 C20 dual fuel tank issue/questions

I have a '75 C20 with dual 20g tanks. I've always had trouble filling up my tanks. Here in CA the fuel nozzles have these very annoying "Vapor collar" thingys that completely block off the filler neck to prevent fuel vapor from escaping. In doing so, they prohibit the tank vent from operating properly, which leads to fuel puking out of the filler neck and onto the ground. That's a pain in the ass all in itself, but recently I noticed the filler tube hose on my aux tank was leaking profusely as well. That led me to crawl under there and inspect the system. Turns out all the rubber fuel lines are in pretty shoddy shape.

I dropped the aux side tank yesterday and am in the process of cleaning it up, replacing the lines, and deciding whether or not i need to replace the sender.

If anyone can help chime in on this question, please do.

After pulling the tank, I found this setup at the top:

Untitled by awesome_adawson, on Flickr

Untitled by awesome_adawson, on Flickr

What exactly am I looking at here? And is it safe/legit/factory? I'm assuming that there's a fuel output line, and then a breather, but what sort of breather contraption is that? and is it safe? Also, if i plan on swapping the sender, is it wise to recreate this exact setup?

Another question, I had planned to relocate the tank vent to a location outside the filler neck, thereby eliminating the issue caused by the CA fuel pump vapor collars. has anyone done this? Would there be any hazard or problem with removing the vent hose from the filler neck and just terminating it somewhere under the bed? It would still allow the fuel vapors to escape, hopefully resulting in successful fueling. Thoughts?


Please let me know if you have any experience in the fuel systems in these trucks... Thanks!
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Old 07-04-2019, 11:10 PM   #2
cadillac_al
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Re: '75 C20 dual fuel tank issue/questions

That plastic vent should allow air to escape the tank no matter what is going on at the gas inlet. I would take that vent hose off and blow through it to see if that vent is passing air. Good luck.
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Old 07-05-2019, 12:47 AM   #3
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Re: '75 C20 dual fuel tank issue/questions

Yes, that is the factory setup for many 75's. Mine has the same thing. The vent is the same as ones on the differentials.

If you attempt to relocate the filler vent, keep in mind these tanks have a large slosh factor. You could wind up dumping a fair amount of gas each time you speed up.
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Old 07-05-2019, 06:07 PM   #4
ShortHaul
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Re: '75 C20 dual fuel tank issue/questions

Quote:
Originally Posted by cadillac_al View Post
That plastic vent should allow air to escape the tank no matter what is going on at the gas inlet. I would take that vent hose off and blow through it to see if that vent is passing air. Good luck.
Copy that, thank you. Yeah it was all pretty gummed up on top of the tank, so I'm not surprised it wasn't venting well...

Quote:
Originally Posted by Dead Parrot View Post
Yes, that is the factory setup for many 75's. Mine has the same thing. The vent is the same as ones on the differentials.

If you attempt to relocate the filler vent, keep in mind these tanks have a large slosh factor. You could wind up dumping a fair amount of gas each time you speed up.
Ok thanks, great to know. I think I've found them on Jegs, although it seems pricey for what it is... I'll keep looking.

Yeah you're probably right about the sloshing... perhaps if i ran the vent tube high enough or at least a long enough hose it'd keep the fuel from making it all the way out the end...

However I also realize that if I disconnect the vent hose from the filler neck then I have a direct hole in the side of the filler neck that would dump fuel on the ground when it backed up into neck upon filling up.

I filled up this morning on the way to work, 15 gallons, and I timed it. It took 11 minutes. Now that's 11 minutes of actual pumping time, with me bent over double, two hands on the fuel pump nozzle, braced against the side of the truck with ever so slight pressure on the nozzle. What a PITA! I'm actually surprised nobody has solved this one yet (or at least nobody's solved it and posted about it...)

Hopefully some new vents will improve filling. We shall see...

The vent i found on Jegs
https://www.jegs.com/i/Performance+T...E&gclsrc=aw.ds

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Old 07-05-2019, 11:36 PM   #5
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Re: '75 C20 dual fuel tank issue/questions

Wonder if you could 'sacrifice' a new gas cap and rip the guts out of it and make a hole large enough for the metal part of the nozzle to go through while holding the vapor capture gizmo clear of the cap. The modified cap would only be for filling and be replaced with a normal one once refueling was finished.

We haven't had to adopt those vapor capture things in OK yet so I haven't seen one in operation.
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Old 07-05-2019, 11:49 PM   #6
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Re: '75 C20 dual fuel tank issue/questions

Quote:
Originally Posted by Dead Parrot View Post
Wonder if you could 'sacrifice' a new gas cap and rip the guts out of it and make a hole large enough for the metal part of the nozzle to go through while holding the vapor capture gizmo clear of the cap. The modified cap would only be for filling and be replaced with a normal one once refueling was finished.

We haven't had to adopt those vapor capture things in OK yet so I haven't seen one in operation.
Hmmmm... I like the way you think... maybe a trip to the junkyard for some cheap gas caps is in order!
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Old 07-05-2019, 11:53 PM   #7
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Re: '75 C20 dual fuel tank issue/questions

https://www.oreillyauto.com/detail/b.../5748039?pos=0.


Here is a 5 pack at Oreillys for $2.69. Not sure what tube size though.
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Old 07-06-2019, 12:28 AM   #8
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Re: '75 C20 dual fuel tank issue/questions

Quote:
Originally Posted by dmack91 View Post
https://www.oreillyauto.com/detail/b.../5748039?pos=0.


Here is a 5 pack at Oreillys for $2.69. Not sure what tube size though.
Thanks! Yeah I found em on rock auto too... same color/make as the original. 3/8" im pretty sure

I wonder if this would work for the filling cap? Gas cap tester. Anybody know if these things are hollow through the center? Couldn't find any pics with it showing all the way through.

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Old 07-12-2019, 02:27 AM   #9
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Re: '75 C20 dual fuel tank issue/questions

Welp... replaced the aux side tank today, cleaned up all the grounds and chased the wires best as I could... and the gauge still reads 3 O clock. thinking it needs a new sender.

Should have just bought the darn thing while I had the tank out. Now its back in and I'm working on the driver's side tank. When its all said and done at least I'll have fresh hoses and the knowledge of how to drop the tanks...
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Old 07-12-2019, 01:21 PM   #10
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Re: '75 C20 dual fuel tank issue/questions

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Originally Posted by ShortHaul View Post
Welp... replaced the aux side tank today, cleaned up all the grounds and chased the wires best as I could... and the gauge still reads 3 O clock. thinking it needs a new sender....
Should always use a ohm meter to check the resistance while the tank is out. Doing so should guarantee that you're putting a working sender unit back in the truck. Do this to new and used senders alike. These squarebodies should sweep between 0-ish and 90-ish ohms when moved between full and empty(or vice versa, I forget).
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Old 07-12-2019, 07:22 PM   #11
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Re: '75 C20 dual fuel tank issue/questions

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Originally Posted by kipps View Post
Should always use a ohm meter to check the resistance while the tank is out. Doing so should guarantee that you're putting a working sender unit back in the truck. Do this to new and used senders alike. These squarebodies should sweep between 0-ish and 90-ish ohms when moved between full and empty(or vice versa, I forget).
Copy... yeah i kinda blew that one... Oh well. Tanks are back in with fresh rubber all around. One of the tanks reads on the gauge so I usually fill em both at once and drive on the one that doesn't read until it runs out or I'm guessing its about to, then ill swap over. Not perfect but its worked so far. I might just go ahead and buy two new senders and do it all over again. Now that I know the sequence its not so hard...
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