TF original gas cap. Vented?

[roger55]

I bought this replacement original style gas cap from Classic Parts of America.



I've put in a new tank under the bed and moved the gas fill to the rear fender. It's the original neck and rubber seal.

The tank builds up some pressure and I'm concerned it might build up too much if this cap isn't vented at all.

Is the cap supposed to be vented? I don't recall pressure building up when I used this cap on the original tank behind the seat.

[Old Crow]

Newer caps are most likely vented in but not out. This allows air in as you use fuel, but also allows pressure to build in tank while the vehicle is parked. I believe this newer design was mandated by the EPA to cut down on evaporative emisssions.
There is some question as to whether or not this is ok on our trucks. I, personally don't see a problem, I've been running a cap like that for 11 or 12 years with no problem. My tank is still in the cab, and since I replaced all the rubber in the fuel system and the filler neck, I don't get any fuel odor in the cab. YMMV.

[roger55]

Newer caps vent in in a vacuum and out too but only at pressure so the EVAP system will work.

My original '55 Bel Air vents at the cap but not by the cap itself but by a little indentation in the lip of the neck where the cap seal sits on it.

My '57 pickup neck doesn't have this indentation but I assume that they must have vented it somehow so the tank didn't over expand.

[1project2many]

Roger...

The original cap had a pressure vent. Turn the cap over and look for a small hole in the center. Insert a pick or cotter pin in the hole and push gently. If you feel somethig applying spring pressure then your cap is vented similar to original. I have found info that the older caps would vent at about 2-3 psi. I have been told replacement caps are built for 5-7 psi. That seems like a substantial difference and if true, you would notice quite a "whoosh" when removing the cap on a hot day.

Keep in mind that the fuel circulating through your system is being heated far more than the original system, non-recirculating, would have done. With heat comes more vaporization and more pressure potential. In an OE EFI system there is a carbon filled canister and purge system which collects and burns vapors created as fuel heats. I installed a power steering cooler on the return line in my EFI retrofitted truck to keep fuel from heating excessively in the tank. It seems to work fairly well for most trips although doing it again I would probably move up to a small trans cooler.

[roger55]

Ah thanks. Just looked at mine. It's a 2 way valve. Lets air in with no restriction but out after some low psi pressure point. So, that's why the get the pressure release when opening but not a concern as the pressure of opening the valve should be a lot lower than would take to expand and damage the tank. Got a poly tank btw.

So no issue here it seems.

However, I see the rubber seal on my cap is already cracking. I bought it a few years ago but the truck has only been driven about 500 miles with it.
Says Mexico on the inside. I wonder if I can get a better quality one?

The photo of the one you show has a knurled edge while mine is smooth. Where did that image come from? Maybe that one has a better seal on it than mine does.

[1project2many]

That image is from Amazon. It's a Stant ACO-3917.

Amazon.com: Stant Mfg. Late Gas Cap - Vented...
Tractor Supply might have caps as well although I wouldn't know how to judge the quality of the seal without using it to see if it lasts. Maybe check out Grainger? They sell sheets of Neoprene rubber. I used one to make the gasket under my combination fuel pump hanger / sending unit.

[roger55]

I looked up that number and it's a cap for a Harley m/c.

Looks like it would fit the truck though. The seal looks different and thicker. Don't know if it would be too thick or not.

Could be the pressure relief is set to a higher psi since m/c tanks are smaller and could handle more pressure than a big tank.

Btw, it doesn't appear that the unit could be taken apart to replace the rubber seal.

[1project2many]

It's just a picture that I found to illustrate the vent location. I wasn't trying to show it as the correct cap. Maybe Stant has dimensions and specifications?

My cap was installed prior to my owning the truck in '93 but it does look similar.

[tmoble]

The one I have/had had the sort of stepped dome with the serrated edge. Worked for me.

The cap that was on it when I got in '84 had a rounded dome, with serrations. Wish I could have found another like that. I never could get to like the stepped dome look.

[capnduane]

Where I live, I need a locking cap, especially when gas gets up to 4 bucks a gallon. I've had this one for a couple of years and it has been good. Seems to have a self venting gasket where the cap sets on the filler neck. Truck runs great and no gas drips down the side of the cab. So I guess I'm good. I think I got it from LMC?