Wasn't aware that there are two different types. Seems I have the other type that doesn't work. Anyone have one that will work for me? Trade??? The one on the left does fit ..but looks janky.

 

What is not working for you?

Your '79 likely has evaporative emissions set up to burn fuel vapors., and uses a non vented cap, the ears set differently as I recall to keep the correct cap on it. They all will let air be sucked in, so the fuel pump doesn't fight a vacuum pulling fuel out ... but yours may not let any escape. I think the diameter of the fill tube is even different. The two extra step leg lugs keep a cap from popping off and fuel rushing out as one removes the cap on a hot day.

My '77 does not have a evaporative emissions set up, and my stock cap was vented letting air in, but would not let air out until a certain pressure level was reached, so mine would sometimes hiss as I opened it (before the gasket cracked). The replacement cap does a better job, not hissing as it has a slightly different valve, but it does work. If I blow hard into it, it shuts like real quick, and it also will stop exit if upside down like in a roll over, but if I real gently like slowly blow, it'll let the air pressure out. It will freely let air in always. My stock one has words "VACUUM" and "PRESSURE" stamped on it. After the gasket cracked it would leak gas if parked full ... and even if not full enough to leak, it let evaporation steal my gas and cause a stink on a hot day. I read somewhere that if left unchecked, it was estimated that up to 30 gallons can evaporate in a years time, explained why I was loosing some.



Sorry, they are blurry. I have been using the Stant in the middle. I like the safety steps, and it never hisses as unless it sees a fast pressure rise, it must let vapors escape maybe as it gets shook as I drive. The MotoRad worked OK, so I have it as a spare, it was cheap and I used it but sometimes it stunk too. Those ribs in the aftermarket caps are pathways for air to get to the valve mechanism on the outside of the gasket / seal point. The original has a different method, maybe a spacer?

I do not think drilling holes is a legitimate fix, but I have seen where others did.

 

Is this on a '79? I thought all the 79 models had fuel doors.

 

I sold my 79😕...this is on my 76 F350.

 

I wondered but relied on the screen name, thought about that painted cap.

 

ahhh...that makes more sense.

I don't think the '76 models had the EEC system to suck gas vapors from tank, into a charcoal canister then into the engine, especially not the F350 since I believe those were emissions exempt a few more years compared to the F100/150 platform. If that is the case, you would need the vented cap to avoid creating a vacuum in the tank.

Regardless Looking at the cap on the right, I see a couple screws. I am wonder if a guy could find a new, vented cap if non-EEC, remove the ears used to open/close the cap, then secure the painted outer cover to that cap. With use of those two screws and some fuel resistant adhesive I think that white cap would stay in place.

 

Thinking about it a bit more, you might need to cut the "new" cap and create some new ears so you have a place to put the screws that line up with existing holes in the painted cap/exterior handle. Couple small cuts, a pair of pliers and a hard flat surface combined with a few hammer taps ought to do it. The larger painted cap should still provide decent enough shield to protect that rubber gasket.

X marks the spot, that may not be the place that you want to cut, but should help with visualizing what I stated above. I would cut those ears in such a way that when the cap is secured in place the outer handle is parallel with the "dent" in the box side. Some wouldn't care about minor details such as that, but it would stick out to me if it was anything but parallel with the body lines. Perpendicular might be alright, but slightly askew would drive me nuts. Yes, I am a bit strange that way...

 

Yes...correct. There were zero emission controls on the truck when it was new. (I was there! ) I do remember hearing the gas fumes venting out through the cap on a warm day...so I'm going to say the tanks are not vented. My hope was to find someone who can use it...or trade for one that I can use.

 

Yeah...I noticed those screws too and thought about swapping parts to make it work but I have nothing to swap. I'm just stuck! Lol

 

As far as I have ever seen, heard, or can imagine .... they all are designed to let air in as gas is pumped out by the fuel pump. The cap that doesn't is defective, dirty, or internally blocked. I have seen this happen on Gold Wing motorcycles so that they only run well with a loose cap, a new cap or long soak in cleaner was the cure then. Vented or non vented refers to the cap being designed to vent pressure build up in some manner or not.

 

Thank you for expanding on that....makes sense.

 

I used to haul mail in one of several '70s F-350s and the main gas cap was on the cab behind the door and was like chrome dome with grippy edge ridges, but I see a recess on your pictures. I guess you want a cap same OD with the cross bar? I looked. seen a few which were chrome with the cross bar and the ones like we had, and some locking ones.

An aside .... Boss man used a 30 gallon saddle tank on them made from two 15 gallon grease drums end to end under the big box body behind the cab on driver side, he also had stretched the frames for those big box bodies, so there was room. When driving, when the truck let you know was time, it hesitated, then we just turned the valve from that tank to in cab tank which had a gauge. Same spot almost every trip, so after a few times, one just knew when.