Gas tank venting and EFI mods

["Mad Max"]

Hiya peeps -



Need clarification. My '78 Ramcharger has a factory 35 gallon plastic gas tank, and I'm converting the rig to EFI.



There are three openings in the tank: One for the sending unit, another in the top front for what I think is the vapor canister vent or fuel filter return(?), and the third is of course the fuel fill.



The sending unit has 4 'openings': the standard 5/16" line, the negative (ground) terminal, the fuel level terminal, and a 1/2" vent that goes to the filler neck.



The vapor vent(?) is a 5/16" hard line reducing via 90* bend to a 1/4" hard line going through the tank via a rubber grommet - not a 'hard' seal.



The fuel fill is simply the gas pump hose opening and the 1/2" vent line (that connects to the sending unit 1/2" line).



Two questions:

1) Is the 1/2" line the vent for both the tank and the fuel fill operation (via the gas cap)?

2) Can the 5/15-1/4" 'vent' line (with the rubber grommet) be used as a return line for an EFI application?



Here's what I've already done:

- Original sending unit 5/16" line replaced with 3/8" line along with incorporation of in-tank 12v 50 psi EFI pump (completed yesterday)



Bottom line: I want to use the vapor vent as the EFI return line, and leave the 1/2" line (for the filler neck) as-is for the vent for the tank (which may already be the case, I'm just not sure, and the factory service manual does not cover the vent process).



Thanks amigos,

- Sam

 

[Philip]

Sam I comverted a 72 Chevy C30 to a Holley ProJection unit (<-POS to) many years ago. What I did was to look for a open place on the sending unit metal mounting plate. I drilled a hole threw and installed a piece of brake line the size I needed. I also angeled it to miss the float while the system was pumping so it wouldn't give a low fuel reading. I then soldered it in.



I had the truck for another 4 years and never had a leak from that area. The injection system was only on the truck 10 months. Of that 10 months it was down for 10 weeks for injection problems.

 

["Mad Max"]

Found it Oo. - just about 1/2 hour ago.



The 1/4" mystery line is part of the 'fuel rollover control system' which is tied into the emissions control system. There's a little 'valve' on the frame (I've seen them before but never know what they were for), and its job is to prevent the flow of fuel, from the mystery line up to the front of the truck, in the event of a rollover. Good idea, and in my case (knowing where I'm taking this particular rig) probably be in my best interest to keep it intact and functional .



This 1/4" line collects evaporated fuel vapor within the tank and directs it into the charcoal canister up in front. That canister vapor is drawn out by the engine vacuum at the intake. Though the book doesn't directly say that this vapor return line is the gas tanks main vent it sure sounds like it to me.



The book only says the 1/2" vent is for fueling operation - no mention of a tank vent (via the cap). In my mind the tank does not need a large vent while driving - no real change of volume is going on (unlike during fueling), so a 1/4" line should be plenty for a tank vent.



So, it looks like I have to utilize the vapor line to the canister and hook it all up, as the tank has no other vent. I will also have to drill a 5/16" hole in the sending unit top plate for the EFI fuel return line. I'm thinking that'll do it - Whadda y'all think?



Thanks fellas,

- S

 

[Philip]

The vapor line will work. Are you removing the cansiter? If not I would install a check valve so it won't push fuel into the canister.