Gas cap keeps getting loose

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Jan 23, 2014
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Hi guys,

I have a 2003 Mitsubishi Diamante and the gas cap keeps getting loose.
So much so that even while driving I can start smelling gas.

I ended up buying a new genuine gas cap but the same thing is happening.

I am thinking that there must be some pressure in the gas tank that gets it undone?
Any ideas what it can be?
 
Joined
Jan 22, 2011
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7,859
Must be some deformality of the gas spout if you already tried an OEM gas cap. There shouldn't be any pressure build up if your EVAP. system is working properly. Pressure is not going to loosen the cap.
 

Spetz

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Must be some deformality of the gas spout if you already tried an OEM gas cap. There shouldn't be any pressure build up if your EVAP. system is working properly. Pressure is not going to loosen the cap.
At times when the cap doesn't get loose, when I open it to fill up I can hear pressure escape, or a vacuum drawing in air.
Is this not meant to be happening?
 
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You shouldn't have pressure build up in your tank. The EVAP system is supposed to "cleanly" vent the pressure.
 
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Owned since new?
Could the filler pipe have been replaced (with an aftermarket cheapie with bad threads) before you got it?
Cheaper plastic can warp with heat.
Any codes? Does your CEL or MIL function?
 
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You shouldn't have pressure build up in your tank. The EVAP system is supposed to "cleanly" vent the pressure.
Some pressure is allowed to build up. If the pressure reaches a level that could damage the tank, it is released through three way valve to the charcoal canister. The canister absorbs HC vapor rather than allow it into the atmosphere to become smog. Similarly while the car is running, a slight vacuum typically exists in the tank due to the purging system. The three-way valve allows air in if the vacuum becomes excessive.
 

Spetz

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I've not owned the car since new, but no nothing in that area has ever been replaced and it is all original.

It does have a purge solenoid, which I might look into as the idle is rough at times too
 
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it is a mechanical device like any. ...you cannot figure out what makes it loosen? is it a threaded screw on type or the older half turn type with locking tabs? In either case pressure or vacuum it should not loosen. Something is deformed or not right where the cap attaches/locks.
 
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it is a mechanical device like any. ...you cannot figure out what makes it loosen? is it a threaded screw on type or the older half turn type with locking tabs? In either case pressure or vacuum it should not loosen. Something is deformed or not right where the cap attaches/locks.
I agree on this one
 
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It's either something inside the filler neck or something on the threads/tabs of the filler cap. I get hissing from the Corvette gas cap every time it opens but just a little, nothing wrong with the gas tank/neck or the cap. Occasionally I smear a tiny bit of grease on the rubber seal on my gas caps, maybe once every couple years.
 

Spetz

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Thanks guys.
I was thinking their may be some cause outside of the cap itself, eg., the tank having too high a pressure or vacuum because of a malfunction.
 
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Could you roughen the cap threads with 100 grit sandpaper?

Not so much that it has an evap leak, but enough to not loosen.
 
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Try an aftermarket cap, maybe it will be out of spec enough to work for you. Maybe the OE cap is clicking before it gets tight.

When the cap comes loose you would think it would lose the seal causing it to throw a code, my old 99 LeSabre did this a few times, CEL would come on and all I had to do was remove it and put it back on. Mech told me to clean the cap and tank neck really good, bingo!
 
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