Brown truck: 78 F150 351m C6 4x4 Non-Catalyst (really!)

In the years I've had the truck, I'd never had a problem that I could trace to the gas caps. However, the gas cap seals were disintegrating, and I decided to replace them.

I've had both tanks out of the truck, and there is no "vapor line" to the front of the truck-consistent with a non ev-em design. The former caps also had two tabs, or ears, which I believe is vented.

Anyway, I replaced the caps, and everything was fine...for a while. One day, leaving work, the truck started, but after a moment, died. I couldn't get it re-started on that tank, but I switched tanks and it started right up. A couple days later, I filled both tanks and drove a few miles home. A couple days later, I drove into town, and it stalled driving down the road. I was able to coast into a parking lot, but could not get the truck restarted. That day, the truck rode home on a wrecker.

I let the truck set for a week, till I had some time to work on it. I cranked it over, and it fired right up. It was fine...for a while. Today, I fired it up, and it ran for only a momenet before it died.

I noticed on several occasions, that when I opened the fuel caps, there was much pressure (or vacuum) on the system, as there was a strong whoosh when I opened the cap. Not sure if it was pressure or vacuum, but that struck me as odd for a vented cap. I pulled the cap, and sucked on the hole, and it did have flow when the seal was loose. But if I held the rubber seal down, there was no flow.

So...I'm thinking the caps are not really vented. So I drilled an 1/8" hole through the center of the cap. It's vented now! I'd rather experience some evaporation than the indignity of another ride on the wrecker. Especially if I'm in Mexico when it quits running. Haven't yet had a chance to test my theory, but does it sound reasonable to y'all?

 

Yes my 74 F250. The rear tank would have a vacuum in it when I pulled the cap. Truck would spit and sputter after awhile. Changed the cap and no more issues.

 

Ive read a couple of times that the new gas caps only vent one direction which causes problems. I have the same problem as you do with new "vented" caps on my 76's rear tank. Ive been thinking of drilling a hole but i dont know how to prevent water from entering the tank when it rains due to the cap being on the side of the bed. Maybe its a non issue with how small the hole would be. That tank builds up some serious pressure in the summer, enough to where it will start spraying fuel if the tank is full due to the heat expansion/pressure not venting.

 

On my truck it was a pain in the butt trying to find the right cap mine was non-vented, I ended up having to go to Napa with the cap and they were able to match it up. If your theory works which it sounds like it should try Napa first for a new cap.

 

Ok, so I filled the tanks several times. My newly vented gas caps (1/8" drill thru center) are now venting. There is no longer any pressure, vacuum, hiss or escaping fuel when I open the cap. So, the caps weren't vented when I bought them, but now they are.

I've seen it posted here that, without venting, it's possible to experience fuel starvation or even a collapsed tank. Neither of those things happened, but my Airtex external electric fuel pump did fail. Maybe the failure was related to the non-vented caps, or maybe it was just time, although it was less than a year old...

 

My 77 always had pressure in the aux tank when I'd remove the cap after driving it a bit but never had a problem from it that I know of anyway. Mine actually took 2 different caps when I changed them, it has been so long I can't remember what the difference was though. It may have been the main cap was vented and the aux wasn't. It was a stock 460 F150 and if it had a vapor return system it was gone when I got it..

 

77 f150 with a 460...I bet that had some get-up-and -go!