Vented Fuel Cap

[p5150]

Yes, I know there is another thread for this but it is locked. I had some info for you all with a blocked vent line like myself.

I went with a MGC-819 fuel cap. You can google it for more information. It has a check valve in it to relieve vacuum and a button on the top to release the positive pressure before you open it.

It cost something like 7 bucks.....

 

[Kckazdude]

Also part # 703-2620 from Napa is the same thing.

 

[tekdeus]

Is this so that your engine bay doesn't smell like fuel when the charcoal canister is removed?

 

[tubbie]

Or... you can just look on the inside of the cap, use a needle nose plier and pull out the black rubber in the middle. Vents the tank just fine.

 

[p5150]

tubbie you missed this: http://www.supramania.com/forums/showthread.php?t=56355

 

[mkIIIman089]

Because there is a new thread for it does not mean it is any less of a bad idea.

IBTL.

 

[p5150]

the fuel cap at the top of the thread keeps the vapors or fuel from escaping out of the filler neck with expansion - a much different proposition and alternative from just removing the seal or charcoal canister and letting it vent.

Of course, the line that used to go to the charcoal canister must be be plugged.

Another reason I went with this is to keep my garage from smelling like fuel.

 

[Kckazdude]

This thread isnt about it being a bad idea or not. Just the availability of the parts for doing it.

 

[jetjock]

Is that cap completely sealed against positive pressure or does crack at some point other than pushing the button?

 

[p5150]

appears to be totally sealed. It cracks with vacuum. I tried to blow into the end of it to get it to crack but I couldnt get it to.

According to Chevron, the vapor pressure of fuel can get up to 15psi, combined with temperature change I would think that the maximum pressure seen inside the tank would be 25 psi under freak circumstances.

 

[jetjock]

I see. Well, keep in mind it's not so much about pressure as surface area. A small delta P across a large area is as dangerous a large delta P across a small area. It'll probably be OK but I dunno for sure. You'll know if it isn't

 

[p5150]

And if the vapor pressure of fuel is 15psi at max, wouldnt the vaporized fuel condense at greater pressures? So with this in mind I think a 25 psi tank would be even more unlikely.

 

[Kckazdude]

To comment more on the cap. The Napa part # doesnt have a button to release the pressure. There is a half turn for pressure relief before it turns out of the filler neck. The pressure has no way of venting without turning the cap when the system is properly capped off.

 

[jetjock]

It'll probably be OK. Consider this though: if the cap were sealed in the opposite direction the vacuum generated in the tank would collapse it. The tank collapses with this small delta P across it because of the surface area involved. The thing is that would happen at a delta much less than 14.7 psi, since that's the maximum achievable (at least on this planet), and there is no way the tank could get near that maximum by having fuel withdrawn from it.

Pressuring the tank is the same thing except the structural aspects of it may be weaker when the situation is reversed. Circular or near circular forms are always stronger in compression than tension. Think of crushing an egg versus the other way around. Even if the tank's form factor is not subject to this it's still going to feel the same force. I guess the real question is would you be comfortable pumping the tank to 10-12 psi with an air compressor. Or that worst case 25 psi.

Will the pressure ever get that high? I dunno. Since the car uses a fuel return type system they'll be heating of the fuel by that in addition to what's done by the ambient, pump, ect. Stock the tank is regulated to + 2 psi by the evap system. In your case it'll be free to do as it likes. I'd be worried about repeatably bulging the tank and the fatigue associated with that. Course, this is all theory. Try it and let us know.

Edit: On second thought the tank is steel. As long as it remains within the elastic limit there should be no fatigue so forget that part.

 

[p5150]

-30 F = 238.5 K
130 F = 327.6 K

p1/t1 = p2/t2

15/238 = p2/327.6 (* both sides by 327.6)

p2 = 20 psi when vapor pressure is 15 psi at -30F and the temperature changes without relief to 130F

1/238 = p2/327.6 (same as above)

p2 w/vapor pressure at 1psi is 1.3 psi

I think it will be fine

 

[jetjock]

That math would make the 2 psi control valve in the evap cannister superfluous. Must be another screw up on Toyota's part

 

[black91turbo]

To comment more on the cap. The Napa part # doesnt have a button to release the pressure. There is a half turn for pressure relief before it turns out of the filler neck. The pressure has no way of venting without turning the cap when the system is properly capped off.

How often would you say to turn it then? (since it has no way of venting when system is capped off)

 

[Doward]

That math would make the 2 psi control valve in the evap cannister superfluous. Must be another screw up on Toyota's part

I doubt it. 15psi of vapor pressure is quite a bit of fuel vapor being released through the cap. Now imagine someone is smoking a cigarette while doing so.

I think Toyota developed a system to provide the fuel pump with 2 psi positive pressure, while keeping fuel vapors at a minimum.

It's quite a nice system, from what I see!