Dual tanks ??

[miked440]

I have a dual tank set up, new six port selector, 3 line sending units, and under stand all the pump and return lines, thanks to you guys older thread history.!! Heres my question were the heck do the 2 vent lines go and what goes on the end??? The guy i got this truck from was clueless and im doing a semi- frame off resto. The chasis is a 80 cab is 75, and i put a 82 front clip on. If any of that matters. I just wanna get the venta run tight so everthing functions proper.
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[BLE 'BURBAN]

You can TEE the vent lines together then they run up to the carbon cannister mounted behind the drivers side headlight.

[Hulk Will Smash]

While we're on the subject, should the fuel guage be reading both tanks? I can't remember how it was on my 77 c20 and my 87 r30 is only showing the fuel level in the left tank.
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[wyattglock]

Originally Posted by Hulk Will Smash

While we're on the subject, should the fuel guage be reading both tanks? I can't remember how it was on my 77 c20 and my 87 r30 is only showing the fuel level in the left tank.
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I just fixed that problem with my 89 R3500. The sending unit was bad on one side of the truck. To answer your question, yes the fuel gauge should read both tanks levels. Another thing to check, get under the truck and have a friend start it and switch the tanks to make sure the selector valve is moving. If not, change that before you drop the tanks for the sending units.

[Hulk Will Smash]

Right on. Thanks man. It pumps fuel out of both sides, I just have to pay attention to my odometer once I switch from my left side to my right. Wasn't going to mess with tracking down a short or bad sender if it wasn't there in the first place though! Much appreciated, thanks again!
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[wyattglock]

Originally Posted by Hulk Will Smash

Right on. Thanks man. It pumps fuel out of both sides, I just have to pay attention to my odometer once I switch from my left side to my right. Wasn't going to mess with tracking down a short or bad sender if it wasn't there in the first place though! Much appreciated, thanks again!
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Not sure if you have ever replaced a sending unit, but if your going to replace the whole thing. A: Stay away from Autozone parts, I have replaced the sending unit twice since July that I have bought from them.
B: Check around, but I have had better luck getting all the pieces separately and assembling the pump and sending unit myself. They are usually cheaper than buying them pre-assembled. One time I saved almost $100 by doing that.
C: Go ahead and replace the fuel filter while you are doing all of this, because you will always have crap in the tank no matter how careful you are not to knock anything into it.

[tucsonjwt]

Check Classic Parts for differential caps - same caps used for vent lines on sending units. They are a light gray color and just press fit inside the rubber fuel line. I ran the extended vent lines inside the frame rails and terminated them above the spare tire - see the gray vent caps. I got 25 feet of Gates rubber fuel line off Amazon for < $20 - cheapest I could find. My truck is in the desert so I think hot weather causes more fuel smell from the vent.

[tucsonjwt]

Regarding sending units, I used Spectra brand replacements and they cause the gas gauge to bounce when the truck is accelerating/braking, but they do provide accurate fuel measurement. I believe stock AC/Delco would not cause the gauge to bounce, but they cost $140-$160 compared to about $40 each for the Spectras.

[Hulk Will Smash]

Originally Posted by tucsonjwt

Regarding sending units, I used Spectra brand replacements and they cause the gas gauge to bounce when the truck is accelerating/braking, but they do provide accurate fuel measurement. I believe stock AC/Delco would not cause the gauge to bounce, but they cost $140-$160 compared to about $40 each for the Spectras.

When your sender crapped out, did the gauge just read empty when you switched over to that tank or did the gauge continue to read the other tank's fuel level?

I'm wondering if I've got a bad sender or bum wiring between the sender and the gauge (like at the switch or between the switch and selector or switch and gauge.

Thanks!

[tucsonjwt]

If you have a vapor canister, then the vent lines connect to that canister. I have an 83 C20 with no canister (since it was considered a heavy duty truck) which were designed to have a short length of fuel line and vent cap installed on the 3rd vent nipples at the sending units. I routed fuel line from the sending unit vents on mine back to the rear bumper area and installed new vent caps. Let me know if you need pics of that setup. I extended the vent lines to eliminate gas smell from entering the cab - now you smell gas when you stand at the rear bumper (if the wind is blowing),

[miked440]

yes id like to see the caps, thats what im curious about. I want to run them some where i just wasnt sure how or what to plug them with.

[miked440]

Awesome !! Where did u get the vent caps
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[tucsonjwt]

Classic Parts, page 115 in catalog, $3.29 each. I would also get one for your differential if it uses a hose and vent, and maybe an extra one in case one pops out of a dried out hose in the future. Vintage Parts might have them also, but I don't know the GM part number, which is necessary if you shop there.

http://www.classicparts.com/1947-91-...ctinfo/93-245/

[miked440]

Thanks
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[Hulk Will Smash]

Yeah, mine just keeps on reading the left tank amount after I switch over to the right tank.

At this point, until I yank the bed or tanks for some other reason, it's not worth the trouble to change the sender (assuming I actually need to). I just wait until it starts acting like it's got low fuel pressure and switch over to the left tank. I always try to keep at least one tank completely full. So I'll run one out, switch to the other, fill up the first right away and then run the second tank empty. Wash, rinse and repeat.

[1tonchevy]

Originally Posted by Hulk Will Smash

Yeah, mine just keeps on reading the left tank amount after I switch over to the right tank.

At this point, until I yank the bed or tanks for some other reason, it's not worth the trouble to change the sender (assuming I actually need to). I just wait until it starts acting like it's got low fuel pressure and switch over to the left tank. I always try to keep at least one tank completely full. So I'll run one out, switch to the other, fill up the first right away and then run the second tank empty. Wash, rinse and repeat.

LOL makes you feel better my C35 only has one tank (duno y) and the gauge dont work lol

[tucsonjwt]

My fuel guage read 1/4 tank on both tanks, until the tank(s) were actually 1/4 tank full, then the gauge worked properly. Following all of the diagnostics I could find, I determined that the sending units were bad. That was not the case - the sending units were OK but the fuel gauge itself was bad. You can find all sorts of diagnostics on fuel guage/sending unit malfunctions both here on this site and elsewhere, but I think there is some guesswork in whatever method you follow.
My advice is always to buy a new fuel gauge for $50 and install it - 20 minute job and you don't have to get dirty. If the new gauge does not fix the problem - sell the gauge (new or working old one) on Ebay or see if you can return to vendor (not likely on electronic parts and the return shipping would make it not worthwhile.)
There are bench tests for fuel gauges and sending units involving resistance checks and special test sets (homemade or otherwise), but for $50 I would take the new gauge route (less than $50 when you resell it.)
I have driven some old squares without operating fuel gauges but it drives me nuts - especially since you don't get too far on these old gas hogs, be it 350 or 454. I cringe every time I drop $100 to fill up the dual 20 gallon tanks on my 83 C20 454 dump conversion. Fortunately, it is not my daily driver - driven only about 1500-2000 miles per year to the dump and Home Depot/Lowes.