Topic: Gas Tank not Venting--Original Pop-Open Gas Cap on '67

[drummingrocks]

Driving back from Mustang Week in my '67, I noticed after stopping for gas that the gas tank was under pressure.  It sounded like air was hissing out of the gas cap area.  The car has its original pop-open gas cap, and when I opened the cap to fill up the tank, a bunch of fumes "whooshed" out, like I was releasing pressure.  The strange thing was that we drove this car almost 600 miles throughout Mustang Week, and never had a problem.  It was a little bit hotter on the drive home than it had been on the rest of the trip, maybe that caused a problem?

Anyway, once I realized how much the fumes were building up, I started stopping every 20 minutes or so and manually opening the gas cap so it could vent.  How is the system supposed to vent with a pop-open cap?  I looked at the cap, but couldn't see any kind of vent that might be clogged.   

[drummingrocks]

Also, the car has always had an extremely annoying tendency to spit back fuel when you're getting gas.  If attempt to put gas in too rapidly, it will spit back all over the car, the bumper, and your shoes.  Could this all be related?

[Bob Gaines]

Also, the car has always had an extremely annoying tendency to spit back fuel when you're getting gas.  If attempt to put gas in too rapidly, it will spit back all over the car, the bumper, and your shoes.  Could this all be related?

The spit back has to do with the shape of the gas filler neck . There are no baffles inside the tank to cause a obstruction which would cause it to spit back more then other times. there are a variety of shapes used 65-70 Mustang/Shelby. The fuel delivery rate has a lot to do with the spit back also . You just have to figure out the sweet spot of how far in to put the pump nozzle into the filler neck where the most gas goes in at the fastest rate, at a given gas pump delivery speed (typically 3 settings). Not much else you can do. Yes the pop open caps are supposed to be vented . Without going into a lot of detail (others who are better typist then I) you may have to do some surgery on the inside of the cap (pin hole) to relieve the pressure if it has gotten stopped up.  Also more then anything else a transverse muffler will heat up the tank and  the fuel to expand and cause undo pressure. More so if not adjusted to factory specs (assembly manuals). If no transverse look for another reason (exhaust pipe too close ). Just some thoughts and observations I am sure there will be others.

[drummingrocks]

Thanks, Bob.  It doesn't have a transverse muffler.  I just took the pop-open cap and the filler neck off, and I couldn't see any kind of vent hole anywhere on the cap.  Maybe I just missed it?  I cleaned the whole cap pretty well, and didn't see anything.

[Bob Gaines]

Thanks, Bob.  It doesn't have a transverse muffler.  I just took the pop-open cap and the filler neck off, and I couldn't see any kind of vent hole anywhere on the cap.  Maybe I just missed it?  I cleaned the whole cap pretty well, and didn't see anything.

I misspoke . The 65/66 caps are vented the 67/68 ones are not in the same way. I think those are pressure relief like a radiator cap. You have to do surgery on those to make them  more vented and unfortunately you can't make them as effective vented as the 65/66. You can add a pin hole for a vent but it will not have check valve like is made into 65/66 cap.

[drummingrocks]

Bob, thanks again.

If anyone has any specific pictures, advice, etc. about how to make this cap vent, I'm all ears.  There's something really messed up with the fuel system as it is right now, and I'm afraid to drive the car until I get it fixed.  I don't really care much for the idea of modding or drilling the original pop-open cap assembly, but if that will fix the problem, then that's how it's going to be.

[Bob Gaines]

Bob, thanks again.

If anyone has any specific pictures, advice, etc. about how to make this cap vent, I'm all ears.  There's something really messed up with the fuel system as it is right now, and I'm afraid to drive the car until I get it fixed.  I don't really care much for the idea of modding or drilling the original pop-open cap assembly, but if that will fix the problem, then that's how it's going to be.

The 69 GT500's had a recall for a problem because the exhaust came out the center right below the gas cap which was vented and in some case ignited vented gas fumes when a backfire occurred. The fix was putting a non vented cap on and then venting the fumes in another location . basically a small brass nipple was added to the gas fillerneck in the trunk and connected to a length of metal fuel line along the taillight panel and then a loop was created in the line and the end vented under the car. See installation sheet. You don't have that kind of problem because exhaust exits far to ether side. Althought the cap is designed to vent when pressure exceeds the set spring tension (like a rad cap) if it makes you feel better this same Shelby fix could be applied to your filler neck. you could also try and drill a small 1/16 hole in the center of the cap like what you find on a 65 or 66 cap but you may get a little fuel weeping out . 

[preaction]

I went through this same issue with my 67 390 cougar, open the gas cap and tank is under pressure. After looking into it I was told that the caps are set up to "vent" air in as gas is used as to not create a vacuum and to not vent out to keep in hydrocarbons.

[rodster]

....... basically a small brass nipple was added to the gas fillerneck in the trunk and connected to a length of metal fuel line along the taillight panel and then a loop was created in the line and the end vented under the car.
........

My 67 Fairlane has this type of venting system from the factory.

[drummingrocks]

basically a small brass nipple was added to the gas fillerneck in the trunk and connected to a length of metal fuel line along the taillight panel and then a loop was created in the line and the end vented under the car. See installation sheet.

I think I'm going to try that approach, adding a vent line.  My only question is how to make it removable, where you can still get the filler neck out through the taillight panel if you ever need to.

[Bob Gaines]

I think I'm going to try that approach, adding a vent line.  My only question is how to make it removable, where you can still get the filler neck out through the taillight panel if you ever need to.

It is a threaded in brass nipple. It is easily taken out if needed. It uses a o ring to seal and not any type of lock tight sealant  etc. to make it permanent. The trick is to start with a small hole and enlarge it using a punch to fold back the metal down inside the hole. That way the threads of the fitting have something to bite on.The 69 GT500's that had this vent recall added to did not have any complaints about gas fumes. FYI the hose that was attached to the metal line that came out the bottom of the inside quarter/trunk floor was routed to a hole in the frame rail so that there is no dangling hose.

[Bob Gaines]

It is a threaded in brass nipple. It is easily taken out if needed. It uses a o ring to seal and not any type of lock tight sealant  etc. to make it permanent. The trick is to start with a small hole and enlarge it using a punch to fold back the metal down inside the hole. That way the threads of the fitting have something to bite on.The 69 GT500's that had this vent recall added to did not have any complaints about gas fumes. FYI the hose that was attached to the metal line that came out the bottom of the inside quarter/trunk floor was routed to a hole in the frame rail so that there is no dangling hose.

Many times dealers did not create a new hole but simply eliminated the factory rubber drip plug and ran the vent hose out that hole.