Road Runner vented fuel cap

[383man]

Its really pretty simple. The tank has to be vented either by a vent line of a vented cap and yes a vented cap lets air in and out just like an open vent line does. Around 1972 they started using non vented caps and charcoal canisters which would store the fuel vapors. If the 69 Runner calls for a non vented cap then the tank has to have a vent line somewhere. If the tank dont get vented to atmosphere the car eventually stop running as the fuel needs atmospheric pressure on it to push it out of the tank. The newer cars of about 71 and up are called non vented caps but at a certain vacum they will vent to let air in and at a certain pressure they will vent to let air out. Thats in case the canister lines would get plugged so the fuel tank wont collaspe. The non vented normally lets fuel vapors out into the charcoal canister and then when the eng is running the canister is purged of the fuel vapor fumes. It can also vent through the canister while the eng runs so atmospheric pressure will get into the tank.
So if your car dont have a charcoal canister which most 71 and older cars dont it has to either have a vent line coming off the tank that usually will go up some and then back down and some even go back and vent inside the uni-body frame rail. They do this so fuel cant just come out the vents on turns and all. If no vent line off the tank on 71 and older then it has to have a vented cap. Ron

 

[Dennis H]

Ron, it's so simple that tons of people have this problem. My 69 has a non-vented cap. Some of this has to be due to the ethanol **** gas we have to use now. Another aspect is generational. If anyone smells anything - anywhere - they are offended and carrying on like fools.

 

[383man]

Dennis my reply was not meant to be offensive to anyone as I simply mean its just simple ethnics that the fuel tank has to be vented in one way or another no matter how crappy our gas is and believe me I know about crap pump gas as I built my 63 to be a pump gas car. My point was the tank has to be vented in one way or another or it just wont work or run to long. Many of the older cars in fact most of them on very hot days will have a smell of fuel vapors and if sitting in a hot garage not well vented it can be worse. I know as my 63 will have the fuel vapors smell sometimes as it uses a vented gas cap. Without running a charcoal canister to catch and store the fuel vapors there will be some fuel vapor smell on hot days when our cars are in not well vented area's. It has to vent the fuel vapors somewhere. The only time you may get around it is with a non vented cap and a non vented fuel tank but the cap will have to vent to pressure and vacum at some low point because as said the fuel simply wont be getting pushed out of the tank if the cap dont vent at least after a small vacum builds or a small pressure.
I dont know the vacum and pressure relief points on the newer non vented caps but some guys may get away running them on non vented tanks as long as they relief at a low point before the fuel stops being pushed out of the tank or before to much pressure builds in the tank on a hot day. Thats about the only way one can avoid fuel smells on real hot days without running a charcoal canister setup. The non vented caps are made for charcoal canister equipped cars but if their relief settings are low enough some may get away using them on non vented tanks. All you could do is try it and it the cars feels like its leaning out after some driving get out and loosen the cap and see if you here a vacum release and then see if it drives ok with the cap off. Ron

 

[resq302]

FWIW - I just replaced one of the new replacement "non vented" gas caps and the rubber started swelling up and deforming. My guess is that the rubber is not that good of a quality and is being attacked by the ethanol or other crappy additives in todays gas.

 

[moparedtn]

FWIW - I just replaced one of the new replacement "non vented" gas caps and the rubber started swelling up and deforming. My guess is that the rubber is not that good of a quality and is being attacked by the ethanol or other crappy additives in todays gas.

Yep, it'll do it.
I've already had today's crap government-mandated ethanol-tinged gas ruin a fuel line and it doesn't take long for it to crud up a Holley, either.
The Edelbrock seems a little more resistant, but I doubt it's ultimately immune, either.
Ethanol robs us of both power and fuel economy, all for the sake of some fictional ozone issue - not to mention, the mandate of a large percentage of the countries' corn crop going to ethanol production has hit us hard in our grocery pockets, too.
I'm sure Archer Daniels Midland loves these laws, but the rest of us have suffered unnecessarily from them.

 

[moparedtn]

...All you could do is try it and it the cars feels like its leaning out after some driving get out and loosen the cap and see if you here a vacum release and then see if it drives ok with the cap off. Ron

That's another reason I put a vented one on mine - the car was starving out on high speed pulls in higher gears. Come to find out, it was an ignition problem. I just haven't switched back to a non-vented cap since is all.

 

[440+6]

Just bought a 1970 vented cap and of course it won't fit so I pulled up the tangs and removed the mechanism, the way it works is it lets air into the tank very easily but it takes allot of pressure to blow air out of the tank, so I disassembled the sucker and cut down the big spring so that ti takes very little pressure to let air out.
Then drilled a hole in a non vented cap, installed the rubber washer, vent and ring, put some pressure on the valve to hold it down and JB welded the ring in place.
I'll let it sit overnight then tomorrow i'm going to a cruise night and I'll fill the tank and see what happens.

 

[moparedtn]

I love a good experiment.

 

[Dennis H]

Just bought a 1970 vented cap and of course it won't fit so I pulled up the tangs and removed the mechanism, the way it works is it lets air into the tank very easily but it takes allot of pressure to blow air out of the tank, so I disassembled the sucker and cut down the big spring so that ti takes very little pressure to let air out.
Then drilled a hole in a non vented cap, installed the rubber washer, vent and ring, put some pressure on the valve to hold it down and JB welded the ring in place.
I'll let it sit overnight then tomorrow i'm going to a cruise night and I'll fill the tank and see what happens.

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If that works, fix one up for me and send it. Just in case.

 

[resq302]

I agree! Keep us informed if it works.

 

[MarPar]

 

[texas69bee]

Here is an article that I found, hope it helps.

Fuel Tank Venting
Your tank has to breathe. In order to breathe, your tank must have a vent somewhere that will relieve both vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down. The fuel level of your tank changes throughout the day, even if you are not driving it.

You cannot put fuel into your tank, unless you can get the air out...and you cannot withdraw fuel from your tank unless you can let air in.

Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.

If you are running a vent line it is important that the line is ran higher than the highest point on the tank including the fuel filler neck. Also, the vent line cannot have a dip in it where fuel or condensation can get trapped in the line. If fuel becomes trapped in the line your tank will then build pressure or vacuum until there is enough pressure to purge the vent which will cause gas and/or odor to come from the vent line. If enough pressure builds up damage could be caused to your tank.

If you are using our remote rollover vent valve # VVR make sure that the vent is mounted vertically. Mounting the vent at an angle may cause the vent to shut off.

 

[Dennis H]

Good article. 4th paragraph nails it. Vent tube routing must be high enough, and not allow moisture to puddle or pool due to the routing. I don't see how a vented cap can help inless it is like the modified one above with a one way valve.

 

[383man]

Here is an article that I found, hope it helps.

Fuel Tank Venting
Your tank has to breathe. In order to breathe, your tank must have a vent somewhere that will relieve both vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down. The fuel level of your tank changes throughout the day, even if you are not driving it.

You cannot put fuel into your tank, unless you can get the air out...and you cannot withdraw fuel from your tank unless you can let air in.

Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.

If you are running a vent line it is important that the line is ran higher than the highest point on the tank including the fuel filler neck. Also, the vent line cannot have a dip in it where fuel or condensation can get trapped in the line. If fuel becomes trapped in the line your tank will then build pressure or vacuum until there is enough pressure to purge the vent which will cause gas and/or odor to come from the vent line. If enough pressure builds up damage could be caused to your tank.

If you are using our remote rollover vent valve # VVR make sure that the vent is mounted vertically. Mounting the vent at an angle may cause the vent to shut off.

Good article. Basically says just what I said the tank has to vent or it wont work. Ron

 

[Richard Cranium]

so to fix this I took my non vented cap and turned it into a vented cap by carefully drilling two 1/8" holes in the outside of the cap with the vented words, careful not to drill through the rubber gasket on the other side. I also added a fuel filter small inline to the top vent line, the one that curls up at the top of the axel. Ever since then I have not had any leaks or any fuel smells or at least with a full tank on a hot day pulling into the garage very little smell if any.

Thanks for the explanation, but it would seem to me that just drilling the cap and not the gasket wouldn't accomplish anything. How would the gas fumes get through the gasket and go out the cap? Can you provide a picture?

 

[HawkRod]

RC,

I had this problem too. My tank is a custom aluminum tank with a fuel pump built in for my fuel injection, but when I drove across country my tank vent barfed lot of gas out. Enough so it dropped my MPG from 17.x to the high 14s.

My solution was as described in this thread: http://www.forbbodiesonly.com/moparforum/threads/fuel-tank-vent-needed.102202/

I'm not saying this is the way to go, but after two fill ups it seems to at the very least have dramatically reduced the issue if not solved it. No, it is not stock, but it seems to work...

Good Luck,

Hawk

 

[bigmanjbmopar]

I remember that post lol what did you go with?

 

[440+6]

Just bought a 1970 vented cap and of course it won't fit so I pulled up the tangs and removed the mechanism, the way it works is it lets air into the tank very easily but it takes allot of pressure to blow air out of the tank, so I disassembled the sucker and cut down the big spring so that ti takes very little pressure to let air out.
Then drilled a hole in a non vented cap, installed the rubber washer, vent and ring, put some pressure on the valve to hold it down and JB welded the ring in place.
I'll let it sit overnight then tomorrow i'm going to a cruise night and I'll fill the tank and see what happens.

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I haven't posted in a while because we had some nice weather ( in the 70's) now we are back to heat wave so...
I have driven the car quite a bit and have kept the tank full and so far not a drop of gas on the ground under any condition, still getting a bit of gas smell if I shut the garage door right away but not near as bad as before.
Looks like the modified cap is working.

 

[bigmanjbmopar]

I just found this on ebay, says its vented and fits a 69 RR? kind of want to buy it but hmmm http://www.ebay.com/itm/New-Fuel-Ga...ash=item54334164db:g:ssIAAOSwbYZXf0bD&vxp=mtr

Has anyone considered retro fitting a vent line on the filler neck? I mean you could easily drill and weld in a 3/8 or even smaller 5/16" tube up to near the cap gas never really rides up the neck that high unless your full throttle all the time from a stop lol

I remembered this tech article from tanks.com Roll over vent is another option http://www.tanksinc.com/index.cfm?ptype=results&Category_ID=160&home_id=-1&mode=cat