How do our fuel tanks vent?

[Greybeard]

Thinking very seriously about using aftermarket injection on my car. I downloaded a PDF installation instruction and it said to "make sure your tank is ventilated". My tank has an added rear sump fuel pickup and my thinking was to use the outlet at the fuel sender unit for my return. But this "vent" thing has me perplexed. Is the tank vented?

[Monza Harry]

GB I believe our tanks use the cap to let air into the tank, and pressure release is dealt with the "Evap' Canister". As for using the old vent outlet for your return read your FI instructions closely. There may be some concern about "Aeration" from the splash, you may however be able to extend the pick up [if yours was "Electric"] or as is for the return with out the screen, [for mechanical pump versions] again check with the manufacturer to be certain. Harry

[cjbiagi]

The only venting from the cap comes when there is a certain amount of pressure or vacuum developed in the tank. If you look at your cap you will see it says "pressure/vacuum". Once a certain amount of pressure or vacuum is detected then the cap releases it, similar in design to a radiator cap. It is otherwise a sealed system. I am not sure how much venting the charcoal canister allows.

[Greybeard]

If my '72 had a charcoal canister, it's certainly long gone. The fuel filler is low, behind the license plate. I once got the cap on crooked, and left a trail of fuel all the way home. The cap doesn't have "vented" stamped on it. However, Dorman sells a replacement level sensor kit that includes the top, no pump. They come with 1,2,or 3 outlets and sell for about $40. My fuel gauge works great so I hate buying another one, but there is an answer. Just gotta drop the tank and look.

[cjbiagi]

Ok, I was more referring to the later Monza's, the early Vega's may indeed be different since that was just the beginning period of emission control.

[gt350fme]

In theory you should be able to use your old tank outlet as a return with the internal pump removed.
As for regular tank venting the outlet is at the very top of the tank goes to the charcole canister that absorbs gas vapors which are inhaled back into the engine when running, if there is no charcole canister, the line that went to the canister will just vent out into the air if it's not caped off.
P.s. And as Clyde mentioned the gas cap often can vent, my new cap vents IN when there's a vacume and not out under pressure.

[monzacoupe]

My canister is long gone, I've got a filter of sorts on the tube that went to the canister to keep dust/dirt out of my tank. Is this all that's needed or should it be sealed off somehow?

Rick

[gt350fme]

I would say you are good, you don't want that blocked off, emissions are another story.

[MonzaRacer]

OK so for Monza gas tank according to one of the guys over on v8monza.com a sending unit/pump from a 94 Buick Park Ave fits into the tank. I have made baffles for tanks with sheet metal and greenlee punches.
Us a cut off wheel to slot tank top and bottom, set holes of around 3/4" spaced about 2 " apart let half a hole stick through bottom of tank. I have even made the baffles V shaped. Slip the baffle in, and tig weld top and bottom.
All it needs to do is restrict fuel from sloshing to back of tank.
The supercharged version of GM pump should handle 350hp easy enough as long as you use heavy 8-10 ga wire from battery to relay and relay to fuel pump.
Another way I have seen to "pump" a tank without burning pump up is making a pocket in top of tank, laying a Bosch 044 pump in there with a check valve inline on pick up side.
I prefer Park Ave idea as you can swap in a Walbro 255 easy enough.