return fuel via vent line

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denny

Master Mechanic
Feb 4, 2017
275
43
got my spare sending today from a forum member here. it answered a question i have seen many times here and other forums. this concerns whether to use the vent line (5/16) as a return. i have read where some high pressure electric pumps will move a ton of fuel back to the tank. i'm talking about the two line sender (feed and vent). i myself was thinking about using this line for a return with the holley blue or other good pump. i have read where people had to install a bigger return because the 5/16 was too small. if you look at the pic you can see that the vent has what i would call a tremendous restriction right under the top. it looks to me that any fuel going back with this line would smack right onto that blockage and create one hell of a splash and fumes inside the tank. the pencil points to the restriction or whatever you would call it. i plan to modify that line by removing that metal block covering the factory return vent and install the line down to the side and under the fuel level. i now understand why this should be vent only but can be modified. more specific my 81 malibu. maybe if that metal covering was not there you would not need a bigger return. lets hear your thoughts. thanks, Denny
 

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pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
27,237
113
Kitchener, Ontario
the vent line on mine had a restiction and check valve that I had to cut off. My efi runs at 43 lbs and the 5/16 at the sender worked fine. I had a 3/8 return line. I ended up finding a new GN tank with a new pump
 
D

denny

Master Mechanic
Feb 4, 2017
275
43
the vent line on mine had a restiction and check valve that I had to cut off. My efi runs at 43 lbs and the 5/16 at the sender worked fine. I had a 3/8 return line. I ended up finding a new GN tank with a new pump
hi pontiacgp. it kinda looks like there are a lot different set ups for different years. my malibu had the canister missing when i got it but the line at the engine compartment looked like it had been blocked off. i think my 5/16 would be ok with a blue pump but a member said his was too small and messed with his pressure because of it. i plan to cut out that plate and extend the tube to the bottom of fuel tank. maybe that plate causes too much back up for a good pump and makes it too difficult to get the right pressure. the vent line was for fumes only and thats probably why the plate is over it at the top. thanks for the thoughts and lets hear some more.
 
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Olds307guy

Not-quite-so-new-guy
Nov 2, 2019
19
3
Hello, now this my kind of thread and ain't no thread Betta than a fuel line thread. I recently did something like this and I'm still having trouble picking up fuel. I mean switching the vent line to pick up line, I'm still having issues picking up fuel. I tested the pump and it checks out fine, so I'll be paying attention to this thread and offer any tips along the way.
 
D

denny

Master Mechanic
Feb 4, 2017
275
43
Hello, now this my kind of thread and ain't no thread Betta than a fuel line thread. I recently did something like this and I'm still having trouble picking up fuel. I mean switching the vent line to pick up line, I'm still having issues picking up fuel. I tested the pump and it checks out fine, so I'll be paying attention to this thread and offer any tips along the way.
thanks for jumping in olds307guy. i think this will help a bunch of us guys that try to fix fuel issues on somewhat of a budget. i had intended to use the vent return line for a regular fuel return. i think it will work with a little modification. i'm glad i got to look at the spare one i just bought to see how it was made. that vent line on mine would not be a very good pickup line. as you can see the vent line stops at the top of the assembly. i would go with the regular pickup line and modify the vent line if needed for a return. hope we get others to chime in. i'm talking about the ones in the pic. there are a few different ones out there. mine is two lines (fuel and vent).
 
D

denny

Master Mechanic
Feb 4, 2017
275
43
EFI=yes
Carb=no
Remove the restrictor either way.
i agree. i noticed i used the term vent line when i should have said fumes. would be interesting to see what it looks like in there when the return fuel rams into that restrictor if left in place and used as a return. thanks, Denny
 
fleming442

fleming442

Geezer
Dec 26, 2013
9,275
113
Well, I can tell you: it will flood the carb by overcoming the needle and seat. I have an EFI pump in the 67, and I couldn't get the regulator below 9psi using the 5/16 return. I had to tee off to a vent port on the rear of the tank (G tanks don't have any) to get the pressure down. I did all this in preparation of eventually going EFI, so all I'll have to do is pull the regulator off and hook up the EFI.
On my G Bonneville, I am using the 5/16 return for EFI, and it still has some quirks that I haven't worked out. If I fill up at the pump with the engine off, it will take 3 tries to get it started. My temporary solution is to leave it running while filling up. I am using the vent as a vent. Before I teed the vent into the airbox, it was straight to the EVAP solenoid in the intake and would shut off in a hard turn when full. I think the Cutlass build will get something different, maybe a Tanks Inc, an Aeromotive Phantom, or a Holley Hydramat- dunno yet.
 
D

denny

Master Mechanic
Feb 4, 2017
275
43
Well, I can tell you: it will flood the carb by overcoming the needle and seat. I have an EFI pump in the 67, and I couldn't get the regulator below 9psi using the 5/16 return. I had to tee off to a vent port on the rear of the tank (G tanks don't have any) to get the pressure down. I did all this in preparation of eventually going EFI, so all I'll have to do is pull the regulator off and hook up the EFI.
On my G Bonneville, I am using the 5/16 return for EFI, and it still has some quirks that I haven't worked out. If I fill up at the pump with the engine off, it will take 3 tries to get it started. My temporary solution is to leave it running while filling up. I am using the vent as a vent. Before I teed the vent into the airbox, it was straight to the EVAP solenoid in the intake and would shut off in a hard turn when full. I think the Cutlass build will get something different, maybe a Tanks Inc, an Aeromotive Phantom, or a Holley Hydramat- dunno yet.
this is great information for the forum members that are starting fuel system upgrades. thank you,,Denny
 
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Olds307guy

Not-quite-so-new-guy
Nov 2, 2019
19
3
this is great information for the forum members that are starting fuel system upgrades. thank you,,Denny
You guys are so far ahead of me, what I mean is that I cannot explain my issues in mechanical talk. Im learning allot by what you guys are saying though. I do have a question about this restrictor, where would it be located in my 79 regal? Im trying to use my vent as supply but didn't get any results. I will try to put 5gal in tank today to see if pump or carb would prime, I can't get car started everything else checks out.
 

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