66 F100, in cab tank ...

Truck came with a locking cap that didn't really fit.

Bought a new vented cap.

Problem is, I've got to leave it loose as pressure builds in the tank and causes a small leak up front at the filter/pump.

If its put on and tightened correctly, and you go to open it, it'll shoot out like champagne cork, and in general, the truck seems to stumble and generally be trickier to start.

Aside from the gas expanding, is there somthing else causing pressure to build ?

 

Are you sure you bought a vented gas cap? The symptom you are experiencing is caused by a non vented cap.

The in-cab tank has no vent, so a vented cap is a must have.

Hot weather will cause gas to expand, so it's a good idea not to fill the tank all the way up, otherwise on hot days, it will seep out past the gas cap.

 

Sounds like your new vented gas cap - isn't.

If it was vented then pressure would not be building up, in my feeble mind anyway.

Also, without a vented cap as you draw fuel from the tank with the engine running then you create a vacuum in the tank and it gets hard for it to draw gas. This might be more the cause of a stumble.

 

I think my "vented" cap is only letting air in, like it's a one way valve.

I would expect the tank to be under negative pressure as there's no tank vent or return line, but every time I check, if the cap's not loose, it wants to come shooting out.

I tried taking the gasket off, but it then it's too loose and gas can splash out while turning right.

Had the truck now for 6 weeks and haven't yet filled it yet past 1/4 of a tank.

Warmest weather we've had is only about 71 degrees.

There's a lot of crud in the tank, and I'm changing the fuel filter every 2 weeks or so.

Would a clogged pump/filter maybe push pressure back towards the tank ?

 

Remember, 71 was a reading probably taken in the shade. If you truck was in the sun it would be hotter. And inside the cab where the gas tank is (assuming stock here) would be even hotter. The vented gas caps that are correct are not a one way vent. They equalize the pressure to the same as atmospheric pressure in both directions.

 

Found something searching the forum here, and looks the proper caps seem to have a deeper collar.

Heading to the parts store tonight to try find one.

 

Let us know what you find and how it works. I'm experiencing the same thing on my 57 Ranchero. The new "vented caps" have a one way valve that lets air in but doesn't let pressure out, it is also labeled as "anti surge" and that may have something to do with it. I ended up drilling a small 1/8" hole in it to let pressure out as a temp fix.

 

B6TZ-9030-A .. In-Cab Fuel Cap-Vented Anti-Surge-Bright Finish (Motorcraft FC-23) / Obsolete

1956/72 F100/750.

GREEN SALES CO. in Cincinnati OH has 4 = 800-543-4959.

KLIMESH MOTOR SALES in Calmar IA has 6 = 563-562-3241.

WESLEY OBSOLETE PARTS in Liberty KY has 8 = 606-787-5293.

CARPENTER NOS OBSOLETE PARTS in Concord NC has 10 = 800-476-9653.
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C4RZ-9030-C .. In-Cab Fuel Cap-Vented-Locking (Motorcraft FC-91) / Obsolete

1951/72 F100/750 / 1949/64 Ford Passenger Cars / 1957/60 Thunderbird / Misc. 1965/70 Ford Passenger Cars.

ANTIQUE AUTO SUPPLY in Arlington TX has 1 = 817-275-2381.

WESLEY OBSOLETE PARTS has 2.

GREEN SALES CO. has 2.

NOS PARTS LTD in Waxahachie TX has 9 = 972-937-2201.

CARPENTER NOS OBSOLETE PARTS has 13.

 

B6TZ-9030-A = Stant 10623 if I was reading correctly.

 

The Stant cap is flat in the center, has a decal on it. The original Ford caps were convex, the later production were Stants.

 

Think I'll just drill a hole , thank you for the replies

 

The nice thing about the correct vented gas caps is if you roll the truck into a ditch on it's left side or upside down a checkball will flop into place to keep gas from pouring out the vented cap. But, hopefully that doesn't happen. It did to one fella though. Check out the pics in this thread.

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

 

I have the in cab tank.i have a new cap..the fuel pressurizes so much it floods engine filling cylinders up with fuel.

 

AFAIK, the filler necks are different between vented and non-vented gas tanks. A vented cap will not work on a filler neck for a non-vented tank truck, and vice versa.

 

I did not know this. Was that true back when, or only more recently? I thought I’d read threads about guys using the wrong, unvented cap, changing to the proper one, and solving their problems?