Gas Cap Hiss
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Joined: Aug 2007
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Gas Cap Hiss
I have a 1996 F150, 300 engine, with the dual tanks. I mainly use the front tank and the cap will hiss when the pressure is released as is normal. My question is: Are both tanks supposed to do this when the cap is turned to remove it or only the one in use at the time? The rear one never makes any noise when I remove it, but then, I never use that one. Who can afford the gas now anyway? I can't find any information anywhere about this. Any ideas or thoughts?
Postmaster
Joined: Aug 2003
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From: St. Peter
The gas cap is designed to release pressure once it get over 1 PSI. I'm assuming you are hearing a hiss from the loss of fuel, creating a vacuum in the tank. I would say you are just fine, being you dont use fuel out of the rear tank. Also the evap system on a 1996 should have the ability to detect leaks, and would set a check engine light if one was present.
Senior User
Joined: Feb 2007
Posts: 223
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From: Kaukauna, WI
I was always under the impression that the cap ISN'T supposed to hiss. Yes it's caused by the vacumm but there shouldn't be negative pressure in your tank that the evap system is supposed to take care of that. Try a new fuel cap. Sounds "hokey" but it'll be a cheap fix.
ALSO, regardless of how much it costs to fill the rear tank, rotate the use of both tanks. I didn't for the last few years and it cost me a new rear tank. If you don't you may run the risk of condensation forming in the tank and developing rust. An "empty" tank is full of vapor and such but changing air temps cause the tank to "breath" and if you ain't using it, the moisture will collect in the tank, thus the rust starting. Even if you don't fill them both, USE them both.
ALSO, regardless of how much it costs to fill the rear tank, rotate the use of both tanks. I didn't for the last few years and it cost me a new rear tank. If you don't you may run the risk of condensation forming in the tank and developing rust. An "empty" tank is full of vapor and such but changing air temps cause the tank to "breath" and if you ain't using it, the moisture will collect in the tank, thus the rust starting. Even if you don't fill them both, USE them both.
Der Postmeister
Joined: Sep 2006
Posts: 6,859
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From: Long Beach, Calimexiforia
My take on cap function was akin to a check valve, allowing air in to alleviate vacuum occurrence but disallow vapor loss. If you intend to ever use the 2nd tank ever in the future or if sold, allow the new owner it usability, alternate tank use. If you intend to drive it til the wheels fall off or something else equally detrimental to it allowing forward motion to continue, lose the second tank. One point in fact everyone would agree on is: They're fairly damn heavy in weight. That's alot of extra miles per gallon you'll gain from now til the multiwheel dysfunction occurs. Something to do while deciding what to do: Switch caps with the other tank!
Last edited by ymeski56; Sep 30, 2008 at 03:36 AM. Reason: ynot
