The fuel tank builds pressure high enough to spit gas from the fill neck when removing gas cap. it is able to spit gas on who ever dares to just pop the cap off with out cracking the seal slowly to bleed the pressure first.
I am kicking around the idea of adding a charcoal canister, purge valve and hoses to burn off vapors and release pressure. All vacuum operated only.
Has anyone preformed such a task? I am looking for ideas and watching for pitfalls.
All information to this subject is appreciated. Thank you.

My question is regarding a 1977 F-150, 2wd, 400, no evap hole in fuel tank (original or 38 gal tank)and no evap system.
I live in Oklahoma.

 

I don't have the correct answer, but in the meantime, just run out there and go ahead and loosen your gas cap enough to where it will not trap pressure, because uncontrolled pressure in fuel system bad! Very bad.
It can mess up mechanical fuel pumps and throw off your carburetor tune, and in worse cases can blow hoses, or even flex the tank in and out repeatedly, until it fatigues and cracks and leaks.

There are several answers though:

Some here have drilled a small hole in their fuel filler caps, while others on the Toyota site I go to have replaced their fuel-logged charcoal canisters. Corollas will sacrifice their tanks when their evap system fails. Probably what blew out my rusty filler neck...

I kind of like the idea of retrofitting a canister purge system, but I think that even the Canister Purge Solenoid Valves they had back in the carb'ed days were still electrically controlled - they were on my '82 and '84 5.0 HO Mustang and Capri. But that's not a big deal, you just need to know where to tap in for power like Henry did it.

So yours seals up that well with the old '77 metal cap? Mine doesn't seal or fasten tight at all, I tried another cap, and same deal. It almost sounds like you have a plastic 'clicker' gas cap and filler neck from the '80s-'90s.

 

JD Q,
I had the same problem on my 77 F-150 that I acquired June 2016 with both OEM dual tanks (aft and behind the seat). I changed the gas caps and both tanks no longer build pressure. I had to order the gas caps from my local NAPA. Unfortunately I don't have there P/N, threw the boxes the gas cap came in. If you visit your local parts store, explain that your 77 F-150 has non-vented tank/s.


PapaBearYuma, from this forum was the individual who I remember pointed me toward replacing my gas caps, good advise that I took and solved me problem.

 

I have a gas cap that allows air in but the cap valve closes to prevent venting out. I will talk to napa as soon as I can. If anyone knows the pn I would appreciate it much.
I know that my lip in the fill neck is shallower than most. This is the lip that the cap hooks to.
I had a cap once that did not seal off and would let gas out around it. The hooks on the cap was too deep.

 

Yeupp...what you need is a vented gas cap. On my '79, I welded up the bunghole on the tank which allowed me to 86 all that Evap junk...Got a vented gas cap and all has been peachy for quite some time.

It's a Stant, #10623, circular ~ 2.5" diameter with only the two ears on it. Can't recall if I got it from JBG on not. Mebbe your parts dude/dude-ettes can cross-reference it.

 

Thanks for all of the information, in such short time. It's good to see all the good help.
Sure beats the TV news!

 

I had a similar problem in one of my tanks. It turned out I had two different caps. The Stant #10749 cap I bought vented both vacuum from the tank emptying and also the positive pressure from heat in the tank. Before I got the correct vented cap I had the filler tubing blow off due to built up pressure. I didn't realize it until I was at the gas station leaking fuel all over the ground!

 

Just re-reading your post, the Stant cap I bought has a long throw on the inside (two sets of two ears at different depth). If your neck is shallower than most then that cap likely wont work for you. Stant should have what you need though.

 

Per http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=160/mode=cat/cat160.htm
Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.
I also read in a separate website that the EPA will not allow a cap to vent to atmosphere.
I am looking into the stant cap stated in an earlier post

 

A "vented" gas cap is a start. Should be available from your local AP store. These have check valves which do hold some pressure or vacuum. Both can be a problem. Some folks drill a strategically placed hole, around 1/16", in the cap to prevent the pressure/vacuum buildup.

 

D8TZ-9030-A .. VENTED non locking Fuel Cap-Use without Evaporative Emission - 2 5/8" O.D. - handle on top of cap-zinc plate finish (Motorcraft FC-807) / Obsolete ~ 12 available NOS

1977/79 F100/350 Stylesides with aft axle and/or midship fuel tank with fuel doors; 1978/79 Bronco.

Give the Motorcraft part number to auto parts store parts clucks, they can cross it over to the brands they sell.

Post #8: Note: The fuel cap with two sets of ears (two above, two below 180 degrees apart) is NON vented, was used with Evaporative Emission.

The fuel filler necks used with this cap are also different. Originally came with a restrictor inside that prevented gas station leaded gas nozzles from fitting in.

Back then, the leaded gas station nozzles were larger in diameter than the unleaded nozzles.

 

Per http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=160/mode=cat/cat160.htm
Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.
I also read in a separate website that the EPA will not allow a cap to vent to atmosphere.
I am looking into the stant cap stated in an earlier post

 

Sorry about the re-post. The battery in my phone passed out during send. Then, when I charged it, the post was sent again.
However, my lock gas cap is not letting tank pressure out. however I have a 1974 Torino Elite to use as a doner.
I plan to drill a hole in the lock cap for now, until I get the charcoal canister & purge valve. I'll then tee off the vent hose on the filler neck to the canister and then on to the intake manifold under the carb. Or maybe to the breather. the vapor will be routed through the purge valve. If the valve is driven by vacuum I will tee from the ported vacuum. If the valve is opened electronically I will run a bee-bee chain from the linkage to a switch. I have a lot to work out still.
Thanks guys

 

I drilled a small hole in the top of the metal filler neck right behind the fender. Mine would build so much pressure it would flex the tank and when I took he cap off to vent it would whoosh air out

 

I would use one OR the other JD, but not both a charcoal canister AND vacuum.
Unless you can control the vacuum and solenoid electronically, just use the simple evap system that would have been in use around '77 anyway.

That's a simple charcoal canister mounted as high as you can get it in the engine compartment or wheel well, and a few hoses.
It really doesn't have to be overly high either, as long as it's above the tank. Even the factory put them on the top of the frame rails near the radiator.
In the older Broncos they wanted them higher up due to the higher probability that the utility vehicle would see more hill and side-hill angles and bouncing around, and the goal is to keep all liquid gas from entering the charcoal canister.
Hence the change to electronically controlled solenoid purge valves after the introduction of computers.

In most cases it was my understanding that an earlier vented cap would not retrofit to a later EVAP-equipped filler neck. But maybe there were oddballs due to the fact that some full size trucks apparently did not get EVAP in all locations until later years.
In lighter vehicles they started in about mid-'70 model year and have been in continuous use since.

For connection reference, the tank vent you create will run to one of the small vent fittings/ports on the canister. Then one of the larger ones will run up to your air cleaner housing so that the mild intake vacuum (not full manifold vacuum) will gently but effectively pull the vapors out of the canister. Then the second large port remains open to atmosphere, but if it's lower mounted it's best to run a hose up higher to keep debris and moisture out of the canister.
If the canister itself is mounted high enough, that port simply stays open. They use what we call a "mushroom cap" that covers the port, but allows ambient pressure in and out.

Or, you can just replace the cap! As some others have done successfully.
I'm a big fan of the evap system myself, and have retrofitted mine with that setup.

Good luck.

Paul