Should gas cap hiss when opened?
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Freshman User
Joined: Apr 2010
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Should gas cap hiss when opened?
'94 F-150 302 dual tanks...both caps have something about "hissing" written on top of them. Like, "carefully open till hissing subsides". Something to that effect anyway. Nonetheless, neither tank hisses on my truck. The reason I'm looking into this is that I'm still having hard starting problems with the 'ol girl. Starts up quickly the 1st time on a given day, the rest of the day it'll crank for about 5 secs or so before it'll fire up. I can usually smell gas as well when it does this and although intermittent, it has this problem more often than never. I've been dealing with this for quite some time now and I've done everything that I know to do: new plugs/cap/rotor, timing set (w/SPOUT unplugged), new FP regulator. Also checked codes and got nuthin', checked fuel pressure w/ engine running and not running and is per spec. This is essentially driving me nuts...the truck runs great otherwise and gets me 16MPG around town. Anyone had this problem and/or have any suggestions on how to remedy? I really can't afford to take it to the dealership!
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
The hissing noise is the gas expanding when it warms up. With engine off, the vapors are held in the tanks and charcoal canister(under hood pass side front frame). When engine is running it will purge the vapors out. I get this alot down here in Florida, where the temps can swing 30 deg from night and day.
Does the fuel pressure stay up with engine off? You may have pressure bleeding down. Also was the fuel filter replaced too?
Does the fuel pressure stay up with engine off? You may have pressure bleeding down. Also was the fuel filter replaced too?
Thread Starter
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Freshman User
Joined: Apr 2010
Posts: 32
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The hissing noise is the gas expanding when it warms up. With engine off, the vapors are held in the tanks and charcoal canister(under hood pass side front frame). When engine is running it will purge the vapors out. I get this alot down here in Florida, where the temps can swing 30 deg from night and day.
Does the fuel pressure stay up with engine off? You may have pressure bleeding down. Also was the fuel filter replaced too?
Does the fuel pressure stay up with engine off? You may have pressure bleeding down. Also was the fuel filter replaced too?
Thread Starter
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Freshman User
Joined: Apr 2010
Posts: 32
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Isn't there a pressure relief valve in each tank as well? I recently patched a small leak in the rear tank, so that may explain why the gas cap wouldn't hiss on that tank. I'm just grabbing at straws here trying to solve my hard-start problem, hoping to grab the right straw! This may not even be a fuel related problem, but something else causing the delayed starts, causing the flooding...thus, the gas smell. BTW, I've also verified the operation of the coolant temp. switch and the intake air temp switch as well.
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