First off I should preface this by saying that I have read through every thread that I can find on the subject and I am not satisfied with an answer to this problem. This is a case for Sherlock Holmes if he were a gear head.

I bought this truck (72 f250 sport custom) in December of last year and it drove great until about three weeks ago when my friend bought a new gas cap for it. The old gas cap rubber seal has deteriorated a lot so we put a new one on it and after driving it for 10 minutes the truck stalled and we drifted over the side the road. When I took the gas cap off, gas rushed out of the filler neck (I took a gas bath). After a day of sitting it still did not start. I put the old cap on and changed points and condenser (for other reasons) and got it running again.

I did some research and bought a vented gas cap. After I filled the gas tank I let the truck sit for a day and noticed gas was dripping off the new cap. After a day of sitting there was still some gas bubbling out the side (note it wasn't hot outside and it wasn't pitched towards the driver side). I put the old gas cap back on again but I am still perplexed why there is so much pressure building in my gas tank. There is no evap system in this model and yet for some reason a vented gas cap did not work and neither does a crappy old gas cap. Thoughts? Concerns? Questions?



-other clues
-aux tank was removed by previous owner who put in straight pipes
- not a vapor lock issue as the carb is getting plenty of gas

 

A common problem with over filling the f250. The vent only lets air in.
A cap basically only seals the sloshing of gas up around the cap to a certain point.
And after 40+ yr of putting a gas nozzle in the fill pipe it scratches the outer lip of the pipes cap sealing surface area. Mr. Watson

These truck came the just a non locking cap until the after market took hold and make more of a universal locking caps..

Orich

 

Not so mysterious. Some '70-'72 trucks had fuel tanks with emission control plumbing. The extra lines went up to a charcoal canister located on the front right-side frame rail. Another hose extended from the charcoal canister up to the air filter housing. This allowed the engine to burn fuel tank vapors. This style of tank used a non-vented gas cap, since the charcoal canister was the vent. Since the '67-'69 trucks had no charcoal canister, they utilized a vented gas cap. The '67-'69 caps are not interchangeable with the '70-'72 versions.

Gasoline expands quite a bit as its temperature increases, gasohol expands even more. Both transition from the liquid to gas phase as temperatures climb, so both expansion of the liquid and gas cause pressure in the unvented tank. Don't overfill the tank, have the correct gas cap.

From Stant, a big supplier of gas caps:

What is a "pre-release" or Pre-Vent fuel cap?
Some Stant fuel caps have a "Pre-Vent" feature. These caps are designed to allow the controlled release of any pressure that may have built up in the fuel tank, so the tank is de-pressurized before the sealing gasket is lifted from the filler neck. This dramatically reduces the possibility of any fuel expulsion from the tank to your hands or clothing.

Are fuel caps for today's cars vented?
Fuel caps have not been fully vented since the 1960's. Modern fuel caps typically have valves that relieve positive and/or negative pressure once it reaches a certain level.

The current Stant numbers for your truck:

1972-1971; OE Equivalent Fuel Cap; W/O Evaporative Emission System .................................................. .................................10623

1972-1971; OE Equivalent Fuel Cap; W/Evaporative Emission System .................................................. ................................. 10702

Hope all this helps!

 

First of all, thank you for responding. This is the first post I made and it's nice to know people read it.

So I don't know if my truck has a charcoal canister or not. What should I be looking for along the right rail? If I do have one, could it be plugged so it's not properly venting?

As for the cap, the "new cap" that I bought is a Stant 10623 and that's the one that was on when I filled it up and a day later gas was dripping out of the filler pipe. Is that the wrong cap if I don't have an evap system?

Is this just a matter of filling the tank up 3/4 and not all the way? It still seems like there is a pressure issue. Thanks for your help...apparently it's not "elementary dear Watson" for me.

 

Bros, Hold on, it's not a 250 problem, or a 70s problem, but all of the older trucks from the 40s & 50s with behind the seat or similar fill tanks.

Do not fill the tank full unless you are going to drive 3 or more gallons out before stopping. Then do not park it fill side low. You can grab an end parking space in a lot and drive the drivers side rear wheel on the curb to make it high.

It's an old issue with an easy work around. I remember gas leaking from my dads truck 60 years ago.



John

 

Fuel definitely expands and contracts, but, three or more gallons!?! Not that bad. Leave some space below the bottom of the filler neck though.

 

Enjoying this same experience in my 69 - I dont' fill the tank unless heading out on the highway and I am even more careful about filling during the warm Texas summer. Sometimes I remember to loosen my gas cap when parking (but that sets up the obvious problem of forgetting to tighten it up). On my 71, it vents down the tube going down the passenger side of the tank, so I use a locking gas cap on it.

 

very common had same issue with my 66 mustang

 

B6TZ-9030-A .. VENTED Fuel Cap-Chrome (Motorcraft FC-23).

Use with in-cab fuel tank: 1956 F100/900 / 1957/66 F100/1100 / 1967/72 F100/750 (except 1970/72 F100 with Evap/Em).

1967/72 F100/350 with factory auxiliary fuel tank (except 1970/72 F100 with Evap/Em).