I was looking to see if anyone else has ever run into this problem. I have had fuel just start leaking out of the gas cap a couple of times. Both times it occurred with the tanks full of fuel and sitting in my driveway on a level surface. One time I open the fuel cap and about another cup or two came out. My truck is a 1979 F150 4x4 that I just restored not long ago. It does have the EEC valves on top of both tanks that I capped off because the charcoal canisters were long gone. I have vented fuel caps on both tanks but they don't seem to be venting. Any help? Could my fuel diverter valve be leaking passed and over fill one tank when drawing fuel from the other tank? I don't think any fuel returns from the mechanical fuel pump since it is only one line from each tank to the diverter valve. I am confused.

 

What fuel caps did you use? I also have a 78 F150 4x4 351m, and I replaced the caps with the "correct" caps from Bronco Graveyard. Turns out they don't really vent, so I drilled an 1/8 hole in the center of them.
Problem solved

 

Evaporative Emission (abbrev: Evap/Em): Unleaded fuel, NON vented fuel caps.

EEC = Electronic Engine Control System aka on board computer. No truck of this vintage has it.

 

Using the hole for the EVAP/EM place a check valve on the hose which allows air in but not out with a vent gas cap, air out under a set pressure threshold, and all your issues should subside.

 

Number Dummy,
Thanks for the correct abbreviation. I just referred to a LMC part catalog where they refer to it as EEC (evaporative emission control).


Do you have any suggestions on how to solve my problem without putting the charcoal canisters can in place? I did install vented caps when I plugged the EVAP/EM valve on top of both tanks but still have the issue. I have one reply to drill holes in the cap but I haven't tried that yet. another replay suggests to put a check valve to allow air in the EVAP.EM line. Is the EVAP/EM valve already a one way valve but to allow air escape to the charcoal canisters?

 

is fuel coming out of both caps or is only one tank/cap doing this?

 

I believe this is a common issue with modern day vented gas caps. I have read others (myself included) having this problem.

The issue seems to be the caps only vent in and not out, so due to thermal expansion of the fuel/vapors it causes either fuel to come out of the cap or to push a bunch of fuel past the carb fuel bowl and down into the intake.

From my reading, the only way to remedy this is with either a period correct vented cap, or to drill a small hole in the center of the gas cap.

 

+1 it is illegal to manufacture an item that vents gas vapors into the atmosphere. In 1977 and before vent away!!

I have seen where someone drilled a hole into the top of the filler tube, under the panel and tapped it with a compression fitting and ran hose to vent.
Just drilling a small hole in the filler hose, on top of course.
One way valve/check valve on EVAP (78-79) tank.
Disassembling the vented gas cap. the list goes on and on.

BOTTOM LINE: Air needs to get in and out

 

I have the OEM vented fuel caps for 1979 F150. I assume these only vent in when fuel is used. Where exactly did you drill the 1/8" hole?

 

Only out of the rear one that I have seen.

 

Couple of techniques:

1. 1/8" hole directly in the top of the cap
2. 1/8" hole in the vent tube on top
3. If you have an EVAP gas tank and the EVAP system is disconnected, just run a short hose and secure it above the gas tank. You can also use a check valve on it so air can go out the EVAP hose and in through the vented gas cap

Hope that helps

 

drilled the 1/8" hole right in the center. Came in from the underside, through the apparent vent opening... After I drilled the hole, I chamfered it with a 1/4" bit