Hey Guys,
So my son has been driving our 1950 F1 & has taken it back & forth to
college - 100 miles away up Interstate 35. Let's call them shake down
runs - He's made it 3 times to date. When he pulled in on Saturday -
we noticed a high pitched pronounced "whistle" coming from the truck. It took a while - but tracked it down to the in-bed gas cap. We opened it to a large rush of air. Hmmm.. I told him to go get a venting gas cap.
The guys at Advanced wouldn't sell him one - told him electric fuel pumps need some vaccum to work & that it was normal.

The gas tank is a 1970 22 gallon mustang gas tank. We've got a 351W with an in-line electric fuel pump. Any thoughts? Zack drove it 100 miles
back up to school on Sunday so it works - I just don't think it's normal to
have pressure build up in a tank like that. The tank has a vent in the
neck that goes off to a charcoal canister. More venting needed?
Leave it alone - He makes it 100 miles down the highway?

Ben in Austin
1950 F1

 

Put on a vented cap . I had a vented cap on a chevy with a carter inline pump. Worked fine ,no problems. The engine ,ford or chevy, won't care. Maybe the parts place were thinking about a late model pump for fuel injection.

 

I'd be checking the vent to be sure it is not clogged somehow. You should not need a vented cap on the Mustang tank if the vent is working properly. Try disconnecting the vacuum hose from the cannister and see if a vacuum builds up...

 

Thanks Vern & Jim,
It's an odd one that's why I asked. It's a loud whistle - almost like a tea kettle. I'll get a vented gas cap & see. We had some lively debates on
whether to put in a "Rollover Vent" while we were getting the gas tank
ready for install. Hmmmm... That would have been an easy $10
solution - Lot's of resistance to it by the "old dirt track racing guy helping me with the build. "If we don't need it for dirt track racing.... You don't need it on this truck....". My advice in hindsight is to ensure you have your tank properly vented - add the $10 rollover vent.

Ben in Austin

 

What controls the draw off the charcoal canister? Usually there are solenoids or vacuum switches. If you are drawing out of the canister all the time, un-metered, you are likely drawing more than the system is designed for, and the cap can't keep up.

A rollover vent valve blocks gas and vapor flow out of the tank when upside down. I don't think it's part of your problem.

 

Hey Ross,
Hmmm.. Our gas tank system isn't super technical. It's just a $100 1970 mustang tank (new) out back between the rails using jniolin's guidelines.
The tank doesn't have a "rollover valve" on it - (I wish it did). The venting is through a tube coming off of fill neck that goes over to a
charcoal canister so it doesn't stink up the garage too bad.

That's it so far - the gas cap is of the non-venting variety & that's what's whistling.

I'm now sitting back & pondering whether I need to add more
venting to the system to allow for a pretty nice size 351W 400HP engine with an electric fuel pump. Is it ok to have pressure in the tank?
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When I looked into rollover valves - the way it was explained was that it had a marble like thing in it & it vented all the time under normal circumstances & in case you rolled over the marble would drop in place
& block the gas from coming out that vent.

Ben in Austin

 

You are correct about the rollover vent valve. That's exactly how they work. Unless you plan on being upside-down a lot, it would be the least of my worries.

A vent with a charcoal canister inline needs to have something to help pull the vapors through. In the OEM application, the other end of the hose coming out of the canister is connected to the air cleaner or carburetor, like a PCV valve. The engine vacuum pulls the vapor and recycles it through the engine.

In hot weather, it's somewhat normal for a non-vented system to build up some pressure and have the cap hiss at you when removing. Especially when the tank is low. I wouldn't expect it to be an issue this time of year. I agree with the others and try a vented cap, just to see what happens if for no other reason. Try a different parts store and just go to the rack and pick one up. No need to complicate the issue getting the counter people involved. My 2c.

 

Hey Wayne,
Thanks for your advice. Well the weather would explain why I haven't heard much whistling up till now. It just turned warm this week - was around 85 yesterday in Texas.

I put the charcoal canister on there after I got some
flak from the female side of the family (2 daughters & my long suffering wife) that our garage sure smells bad with that horrible truck in there - wouldn't it be nice if you made it go away.... The charcoal thing was my attempt at making it go away & I didn't have it draw off the engine & circulate back up front - (too complicated for me). I fix one thing & now
it pops up somewhere else.

Ben in Austin

 

Charcoal canisters have a check valve in them, on the top where the tubing connects. They won't let air INTO the tank, they will only absorb vapors coming OUT. Wayne is right, you need to suck the vapors out ("canister purge" in OEM terms) or the charcoal will become saturated and soggy. But OEM's have controls so it only sucks out of the canister once the engine has started. A vented cap should work but to keep the smells out of the garage, you just need a few more pieces.

 

I've been wrong before, but I think vented gas caps only let air flow into the tank to replace the fuel that is consumed. I don't think those types of caps will let pressure out. It would be odd to have pressure built up after driving 100 miles....so let me ask this....are you sure the pressure surge was coming out of the tank, and not a rush of air into the tank (i.e. vacuum build up as a result of the fuel system sucking fuel out of the tank for 100 miles). If that's the case....get a vented fuel cap.

Dan

 

Ben, my thoughts. The guys at the parts store are FOS, enough said. The dirt racer may not have a rollover vent, but he's got a vent some where or he runs really short races. The vent you have on the filler neck works the same as a roll over vent, just with out the roll over part. The roll over part just keeps gas from leaking out if you roll the vehicle, other wise it's just a vent. My tank is vented with a rollover valve and 3/8 rubber fuel line running outside the cab to a fuel filter on the end to keep dust from entering. It works just fine, and I have a non vented cap.
So I think your problem is the charcoal canister at the end of the vent line. Like Ross said, it's for catching fumes leaving the tank, not venting into the tank. The simple test would be to pull the line from the canister and see if that solves the problem. If so, cap the line with a filter. For every gallon of fuel out of the tank, there's a gallon of air that must replace it.

 

Yes, you should determine if the air rushes in or out when you open the cap.

 

My miss for not explaining well - the rush of air is definitely into
the tank. It's like all the air has been sucked out of the tank &
when you release the cap it rushes in (but a lot more than any gas tank I've ever experienced). I'll go to a vented cap for the short term -
I'll need to re-think the gas fumes into the garage problem again.
I think another vent to the filter is a good idea & I'll deep six the charcoal canister it's obviously not helping. Thanks guys.

Ben in Austin

 

My additional 2c,
If the air was rushing into the tank when removing the cap after a long road trip, I'll say it sounds more like your venting system isn't letting enough air into the tank to replace the space the fuel was taking up as it was being emptied. You're lucky the engine didn't stall leaving your kid stranded somewhere from lack of fuel delivery. That's a common problem we've seen here many times from this exact situation. If you add a vented cap and do nothing else, my guess is it'll be problem solved.

 

52 Merc is right. I can tell you from experience that it can leave you stranded. The quick fix was to drill a small hole (1/8" or less) through the middle of the cap rivet. That worked 100% and even in a roll over wasn't big enough to let out much gas.

Of course the correct fix is to make sure that you have the right emission plumbing and that it is working correctly. I'm not sure about the 1970 Mustang but on the '70s Jeeps the charcoal canister purge solenoid was run by vacuum from a port halfway up the carb body not on manifold vacuum.

My other concern based upon what you have told us is that the your wife and daughters thought that the gas odor was objectionable. They may be more sensitive but I would be concerned that maybe something else is wrong that is causing the odor. We almost burned down Virginia City many years ago with a development vehicle that had a mis-located charcoal canister. It has to be above the highest level of fuel in the tank or it can act like a siphon if the check valves are not working properly in the system.

The small hole drilled in the cap should fix it temporarily but I would find the real source of the problem.