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Fuel cap venting

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Old 03-28-2012, 03:30 PM   #1
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Vehicle: 1986 Mustang GT (5.0 - engine code M)
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Unhappy Fuel cap venting

I just replaced the fuel pump in my 86 mustang GT and now it's doing something strange that it didn't do before. I runs fine, no strange noises, etc, but when I take the gas cap off right after running it (especially if I've had the A/C on), there is a very strong hissing noise and the strong smell of gas. When it does it's hissing sound, I don't know if it's sucking air in or expelling air. What's up with this??? Is it dangerous????

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Old 03-28-2012, 03:37 PM   #2
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Vapor lock inside the tank. Vent tube on the top of the tank is clogged/pinched/melted.
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Old 03-28-2012, 04:34 PM   #3
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True that older cars like ours run vented, but newer cars are sealed and do this normally thanks to emissions regulations. However since your system isn't designed to do this, there are risks, make sure it isn't collapsing your tank
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Old 03-28-2012, 07:52 PM   #4
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HISSSSS.......Cobra in the tank!
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Old 03-29-2012, 09:07 AM   #5
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How do I test to see if the vent tube is bad without dropping the gas tank again?

If the problem is a vapor lock, how do I fix that?

Or is fixing either problem involve dropping the tank and replacing the vent tube?
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Old 03-29-2012, 09:52 AM   #6
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What would happen if I took the vent tube from the tank that runs to the charcoal canister and instead of running it to the canister, splice it directly into the line that runs to the PCV valve (which is where the trapped vapors in the charcoal canister go anyway?
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Old 03-29-2012, 10:16 AM   #7
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The charcoal canister vapors go into the intake manifold and go into the airstream to be burned like normal fuel. They do not and should not go into the crankcase.
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Old 03-29-2012, 11:52 AM   #8
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If it wasn't doing this before you dropped the tank to replace the fuel pump, I'd be willing to bet you pinched the vent hose somewhere above the tank when reinstalling the tank... so simply bypassing the charcoal cannister and running the hose directly to the intake won't solve your problem since the cannister, more than likely, isn't your problem either.

I'd unhook the vent hose at the cannister and put a vacuum gauge on it to see of it holds vacuum... if it does, you have it pinched above the tank. It should flow freely and NOT hold vacuum... if it's flowing fine, THEN you have a cannister problem... and actually, it probably isn't the cannister, it's probably the solenoids that open and close the flow from the cannister to the intake. They are either not working or not getting a signal from the ECU, or you have a clog somewhere in that general area.

But once again, I'm going with YOU caused this problem by pinching the vent hose since this problem presented itself only after you dropped the tank...and now you're faced with dropping the tank again and correcting the issue.
'86 Mercury Capri 5.0, 5 spd, mildly modded.
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Old 03-29-2012, 10:47 PM   #9
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Talking Fuel cap venting noise

Thanks to all who answered my question!!!

I inspected the area by the gas tank and SURE ENOUGH!!! I pinched the vent line above the gas tank against the car body. I dropped the tank about 1 inch, moved the hose out of the way and that fixed the problem!!! The whole thing took about 1 hour.

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Old 06-16-2013, 07:12 PM   #10
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Couldn't one just get a vented gas cap and call it a day? .. ....serious though.. :/
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Old 06-16-2013, 07:12 PM
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