I have a 1964 Ford F100. Just in the last few days I put a new mechanical fuel pump in, fuel filter, fuel lines, gas cap. All was well. Truck runs great. Today I was adjusting the clutch and noted gas dripping from the fuel pump. Truck had not been run since the day before, at least 18/20 hours since it last ran. It was warm out, in the upper 80's, the truck was sitting in the sun. It appears it was leaking around the inlet from the tank. I tightened the fuel lines, inlet nipple, seemed OK. I took the gas cap off and there was a great deal of pressure that blew from the tank when I loosened and removed the cap. Fuel pump appeared to stop leaking. I put the cap back on, about 15 minutes later it started leaking again. I took the cap off again but this time, no pressure build up noted blowing from the tank as it did previously. As the shade over took the truck, it cooled off, pump stopped leaking. So ... why? What can be done to prevent this from happening? Never experienced this in the 45 years I have owned the truck.

 

Vented gas cap? Try putting the old one back on, see what happens.

 

That would be a great idea ... if I had not already tossed the old cap. Appreciate the response.

 

From past experience I'm guessing you need a vented gas cap.

 

You need one of these. Gas Cap for 1949-72 Ford Trucks and Cars | Dennis Carpenter Ford Restorations (dennis-carpenter.com) Or drill a 1/16 hole in the check valve of the cap you have.

 

Stant cap

From O'Reilly Auto Parts. I will order the other cap, in the mean time where would I drill a hole in this? And how big?

 

Drill thru any one of those 5 little holes with a 1/16 drill bit. Just go thru the plastic check valve not the entire cap. Or you can bend those tabs up and take it apart and do it. The cap you pictured is only vented in and not out. It is for a vehicle that has a vapor recovery system.

 

Caps vented to the outside have been illegal since the early 70's. Drilling a hole as mentioned is about the only solution..

 

Thank you, I drilled it out and it appears that it is no longer leaking. Truck is in the sun and on checking from time to time it does not seem to have the blow the cap outta your hand pressure so far. I have ordered the cap from DC. Appreciate the advice!

 

Just to add one thing in case someone doing a search runs across this. Having the wrong cap in your case caused a fuel line leak as you discovered. But more often what happens is the pressure in the tank builds up to the point that it overpowers the needle and seat in the carburetor. When that happens, the tank relieves pressure by dripping fuel into the engine. Not a good thing.

 

Having the correct 2-way vented cap is not a terrible idea since if you do happen to roll the truck on its side (which we've seen happen to forum members like the link below) the correct caps have a check ball that drops into place when the cap is inverted - thus keeping gas in the tank rather than running all over the ground and possibly igniting a ground fire in very close proximity to the gas tank. It also should help keep water from getting into the tank if you roll over into a wet ditch like this where the cap is underwater, which happened to Jose the F100.

Engine stalls at idle and at full throttle - Ford Truck Enthusiasts Forums (ford-trucks.com)