Today is the second time the liquid fuel has come out of the filler neck and run down the side of my 67 F100 Ranger Camper Special ruining the paint.

I thought I had it figured out after the first time it happened. I tried to not park the truck with a full or near full fuel tank and to leave the fuel filler cap ajar. I leave it about a half turn open so that it just sits loosely on the filler neck.

That is how it was today, but once the tank got hot the fuel came out again. I think it is being pushed out by the pressure created by the fuel evaporating in the tank in the heat of the day. If the fuel level is too high I think the vapor pressure causes it to come out of the filler neck. When I took the cap off more liquid fuel came out then there was a burping action as the liquid fuel went back down the filler neck.

I don't see any vent line on the tank...is the cap supposed to do all the pressure venting?

 

Do you have a vented cap?

I had similar problem even with my tank vent. If I went around a corner hard fuel would spill out the cap. I got a new cap and that solved that.

 

The cap looks like it is vented. It has a small pin hole in the middle and when I shake it I can hear something rattle. Maybe I just need a new one.

But if the cap being left half on doesn't stop the pressure build up I don't think a new cap will.

 

Try a new cap. Oreilly part number 6024 I believe.

 

Thanks, I'll give it a try before drilling a hole in the top of the tank to add a vent liine.

 

I wouldn't add a vent line. Vent to where? Add in a charcoal canister? Like a later Bump? If you do just don't blow yourself up!

 

Mine vents out the passenger side floor. Just an open tube. I suppose it had something on it at one time but not anymore.

 

I'm no expert on these trucks but if I had this problem I'd most likely install a vent and vent it directly to the air filter housing.

 

That's an alright idea....

 

Some '70-'72 trucks came with evaporative emissions system, in-cab fuel tanks. The tanks had a vent line coming off of the top right of the tank that ran through a hole in the cab floor to a charcoal canister mounted on the right frame rail, near the core support. You could locate one of these tanks and install it but I suspect you have a faulty vented cap.
FWIW, the vented tanks used a different filler neck that accepted a non-vented cap but would not accept a vented cap (and vice-versa.)
http://www.fordification.com/tech/im...cab-w-evap.jpg

 

Thanks, to add, do not "drill a vent". Can't you just swap the pickup/sender assembly for one with a vent?

Then add a steel 3/16(brake) line from the tank area to the firewall area and link it with a small vacuum hose.

Over the years I've removed many charcoal canisters and just connected the vent line to the air filter, never had an issue.

 

Mine has done that too, and on fresh paint but it didn't hurt it.......that would have been bad. Going to look into a new cap myself...

 

Here's another thing to think about also. I had the same issue on my '63 when I was a kid. A lot of the time when the filler neck is installed through the cab hole and into the tank, the filler neck protrudes into the tank, well past the opening of the tank. When the tank is close to full and the fuel expands on a hot day, the fuel expands into the filler neck because the filler neck is below the opening of the tank. The air that is trapped in the tank above the bottom of the filler neck is now trapped because the fuel level has expanded into the filler neck. As the trapped air expands from the heat it pushes the fuel out of the filler neck.
The way that I cured this on my '63 was by drilling holes in the side of the filler tube at the point just below where the filler tube was inside the filler neck of the tank. This prevented air from becoming trapped in the tank.
Does this make sense?

 

^ I understand it, bobby.

 

Yes, that sounds like what is going on. The fuel is able to block off the filler neck so that the air pressure can't escape. I'll take a look at how far the filler neck goes into the tank tonight...or maybe I should just start using the auxillary tank.