Gas Caps for Dummies

[TallGMC]

Hi All,
Newbie here and love this site! I dont have much experience with mechanics but could use some help. I ordered a new gas cap and it doesnt really fit. It just sits on the filler neck. It will lock but some days it doesnt seal because I can lift the edge and wont hear the vapor hiss. I have been reading about how there are vented and non vented caps/tanks. How do I know what I have and what I need. I have attached a pic for you to see the current cap. Help me out! Thanks!!!

[Southcity]

These old trucks that still have the original gas tank tend to have worn tabs inside the filler neck (the metal ears that sit inside the filler neck that the gas caps latch and lock on to) so you will never get a real good air tight seal because of it, whether it's an original cap or a new one. What I did was buy two cheap caps at a local parts store, pulled the rubber ring of one and added it to the other to help in sealing and if that's not enough, you can even trim a piece of dense flexible foam pad (about 1/4 inch thick in my case) the same shape as the rubber ring, slip it on the cap, place the rubber seal over that and install. Hope this helps, I'm sure others will chime in with their fixes.

[cdowns]

if you ever remove a gas cap and you here the woosh of gas pressure you have a problem// for gas tanks to work properly they must in some shape or form have a vent system weather by a vented gascap or by a vent hose connected to the tank
67-70 had vented caps 71 72 had ventlines from tank to charcoal cannisters to the carb

[TallGMC]

Originally Posted by cdowns

if you ever remove a gas cap and you here the woosh of gas pressure you have a problem// for gas tanks to work properly they must in some shape or form have a vent system weather by a vented gascap or by a vent hose connected to the tank
67-70 had vented caps 71 72 had ventlines from tank to charcoal cannisters to the carb

Thanks for the replys

So I guess my question is how do I know if I have a vented gas cap?

Also is how the cap fits on the neck an issue?

[dmack91]

My locking cap is an older Stant and it says inside "vented" (center raised portion that goes in the filler neck).

I couldn't find an extra gasket, but found a scrap of radiator hose that fit perfect. I cut a piece about 3/8" thick and I haven't leaked on a corner since.

[TallGMC]

Next logical question would be, what would happen if you have a non vented gas cap where you are supposed to have a vented gas cap?

[ChevLoRay]

One of the hardest-to-find items for one of these trucks is an original gas cap. I still have mine, but I replaced it on 9/11/01. I had a feeling that gasoline might get hard to get, or even be rationed, not knowing what was going on with our nation on that day.

Oh, turned out that we didn't need to worry about gasoline.

[dmack91]

Originally Posted by TallGMC

Next logical question would be, what would happen if you have a non vented gas cap where you are supposed to have a vented gas cap?

The pressure has to go somewhere. If the cap isn't vented and sealed tightly, the buildup of pressure will find the path of least resistance to escape. The only 2 options I can think of are the gas cap seal and the sending unit seal. One will have to give.

[Alan's Classic]

I installed a side fill in my bed that is a 99 Ranger door, cap and fill tube. Every time I take the cap off there is pressure from expansion.

I'm running a regular SBC fuel pump and no return. Is there any possible way that the pressure can reach the carb?

I'm thinking of drilling a very small hole in the cap to vent the pressure. Is that a good idea?

[ole dollor]

Alan's classic: yes the gas tank pressure can effect the fuel pump pressure, and cause it to flood, it seams like its worst when the motor is hot, and then shut off for a while ,and then when you try to restart it shows to be flooded.
i would not drill a hole in the cap. if it builds up pressure when you are full, it could leak or spew full out of it. it would be better to get a vented cap.
but a vented cap i guess could do the same thing unless they have a ball or flapper in them so fuel would not leak out but air could. i have a locking cap on ole dollar, and still have the old cap to, i will check tomorrow, and see what they do...good question. anyway i have had two different pickups that i had to put a three line fuel pump on to make them stop flooding, and i tried every thing i could think of and the three line pump was the only way i stopped them from flooding and running rich. the return line to the tank will take pressure off the needle and seat, and send the extra pressure back to the tank, one pickup already had the return line and one i put a nipple in the top of the tank and plumbed it in. long answer but i hope it helps...

[Alan's Classic]

Appreciate the input. I'll look into venting the pressure. I don't believe a vented cap will be available for a 99 Ranger filler.

[ole dollor]

alan's classic: i finally remembered to check my gas caps, my locking gas cap will let air in but not out, and i cant find my other one at this time, not sure what i did with it...but im sure a thicker gasket would help with your problem...