Looking for a locking vented gas cap for my 1979 f250. Truck had emissions on it, previous owener removed everything. I have taken the last cap off and felt a whoosh of air, so I believe the po has the emissions on top of the tank plugged correctly. meaning I need a vented cap.


I don’t know if the last cap I had was vented or not. I’ve found this which resembles my last locking model.
https://www.oreillyauto.com/detail/b...rd/f-150?pos=3

but it’s kinda rinky, I actually tore mine off with my hand after losing my key last week. Oreillys wants $12 shipping plus the purchase price.


Stant makes a couple good looking metal locking caps but I can’t get a clear answer on their website as far as vented/not. Rock auto lists Stants products as being “emission control”.


Im really interested in a motorcraft locking cap. But can’t seem to find one a part number for one that’s locking and vented. Any help would be appreciated.


If I can’t find a locking vented cap I’d be interested in making an emissions cap vented, without letting gas evaporate out. But I don’t know how to do about doing that.

anyone know of a locking vented motorcraft cap or or quality locking vented cap at all?

 

I don't know this for a fact regarding the '79's but many of them are not interchangeable for emissions reasons. In other words they didn't make a vented cap that would fit your filler neck. So you had to go to plan-B and drill a tiny hole in your cap instead.
They actually are "vented" in a way of course, allowing air to flow inward (some caps, but perhaps not all) through a 1-way valve. But not allowing the venting to go outward and allowing fumes to escape.
Maybe over the years someone has come up with a cap that both fits, and has the old 2-way venting. But I have not heard of it.
On the Broncos the difference was in the reach of the two fingers that locked into the neck. One cap just was not going to fit the other neck and vice versa. Hence the workaround of drilling the hole.

I still don't understand why so many people yank off those thoroughly beneficial items. Takes zero horsepower from your engine, but keeps the fumes out of your garage. Guess not everyone appreciates the extra parts and plumbing. But I leave all of mine intact along with the PCV valves. Heck, nowadays I leave it all including air pumps and egr valves!

Good luck.

Paul

 

I suppose a secondary work around, and likely the better choice, would be to re-instate some sort of venting. Short of reinstalling a full charcoal canister setup (which I've done many times when it was removed, or when wanting to upgrade to a newer design for instance) you can simply remove any caps on the vent line fittings at the tank(s) and run a hose up high under the bed and either put a couple of loops in it, put a 1-way flapper valve in it (like from a transfer case or other gearbox) or one of those little K&N type filters in the end.
This would not completely eliminate fumes, but would keep them reduced without totally blocking off the venting.

Paul

 

That makes sense, hadn’t thought of it from that angle. Searched and couldn’t find a canister for sale anywhere, I can’t install a factory air cleaner on my truck do to my carb. Look like I’ll be adding a vent in probably.

So vented should mean two way vented, in and out? To allow air in, and pressure out. Where as the emissions caps only allow air in, no escape out the cap.

For kicks and because I still need a cap I ordered the plastic one from oreillys. It says vented in the specifications so I guess I’ll see how it goes. I’ll investigate on it how to drill a hole if I can.


So if i installed a vent on one of the top hoses and ran it up high under the bed, that would fix any vacuum/pressure problems in the tank? Or would the kind of vent described only allow air out? Still pulling a vacuum when the truck was running. I guess s cap that would allow air to flow in would fix this. In at the filler neck, vapor out the top hose.

 

The vent should work fine as long as there is no 1way valve in some kind of fitting. An anti-rollover valve should not cause trouble as just venting should not be enough to make the valve close on it's own. But an old defective one could cause some trouble I suppose.
And just to be clear, that woosh of air you felt was coming out? Or was it by any chance sucking inward? If sucking then your cap is probably bad and any new one should fix it. I would think anyway. Pretty sure they did not have fully sealed caps in '79 so were not relying 100% on the evap system to let the tank vent while emptying. That is somethign that probably came about (if it did at all?) way into the computer controlled era.
If it was wooshing out though, then it's a 99% bet that the blocked off evap vents are the culprit. Heat expansion of the fuel was one of the things they were trying to control of course. Forcing the fumes to go into the charcoal medium rather than out the old style caps made a big difference, and by itself should be sufficient to let enough air in to compensate for the engine running.
So yeah, I'm pretty sure that your new vent alone, whatever type you decide to go with, would be enough. It won't hurt that the cap is also part of the system, but I'm thinking that either one alone would work. Just that both would be better.

And charcoal canisters are sold by the bushel and are available at every Autozone, O'Reilley's and Summit and Jegs type outfit. Very common thing these days. Even finding the "mushroom caps" the (vent covers to keep debris out) are available again.
While the '79 did somehow manage to factor engine vacuum into the equation, if I was rigging one up I'd do it like the earlier years and forget the vacuum and just vent it into the air cleaner. This would be set up like this:

1. Small hose from the tank(s) to the small fitting/nipple on the canister.
2. If your carb is still stock with the bowl-vent, then the medium size fitting on the canister has a hose run from it to the hose fitting on top of the carb.
3. One large port on the canister has a hose running up to the air cleaner housing anywhere that's convenient. No vacuum, just a gentle pulling into the carb. If your air cleaner is open then this could be a problem. If it's a stock housing, it's an easy fix. If there is a cold-air intake snorkel in place you can even plumb it into the snorkel to be sucked in with normal intake air. That's how I'm going to do my '68 Bronco in fact.
4. Second large port just gets a "mushroom cap" that leave it open to ambient air pressure but protects it from water and debris from getting in.

The original canister on a '79 would be mounted on top of the passenger side frame rail just behind the radiator and underneath the air cleaner snorkel.

Paul

 

I think it used to suck in. I say that because I think if it was blowing out I’d smell more vapor or possibly feel warm vapor. Instead it’s like a light breeze across my knuckles with the whooosh sound. Yes a rollover valve would be a great idea for the vent hose.


I have an edelbrock carb on this truck, so no provision for routing the can to the engine. If the engine side could be routed anywhere else I’d do that.

So, off your first paragraph talking about the cap if the cap doesn’t allow air to flow in and the cap is described at vented there was a problem with it.



So, I need either to hookup a canister or vent the tank to atmosphere. The cap should let air in but not out, vapor goes out either the vent hose or into the canister. Sounds very simple now. I didn’t realize that’s all the canister did. My previous parts truck had one still. Another poor mod be the previous owner.


If I can’t hook the canister up to my edelbrock I’m gonna mount the vent hose in a dust proof little box under a bed side.

 

Sounds good. Let us know how it goes.
Maybe I'm wrong about the caps venting inward even that far back, and perhaps they did rely solely on the venting system. But from everything I've heard the caps were still vented inward for the reasons we've been discussing. Or maybe they're just the bare minimum and your Edelbrock is sucking more gas than the cap can handle!
How's your mileage...

Just kidding. I'd think it would be enough. And besides, we're lucky to get more than 8mpg around town anyway, so that can be a lot of suckage!
Either way, the vent hoses are pretty minimal in size. Maybe even just 3/16 instead of 1/4? Not sure, but it would seem from that size that we don't need a ton of venting capacity. Just the single hose and whatever the cap is capable of.

I'd hate to see what's in a modern '96 or newer vehicle!

Paul