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1966 Mustang Hardtop 289 4 Speed
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Discussion Starter #1
What are you using to prevent the top of your stock fuel tank from being sucked in? Just realizing now that 65-66 stock fuel caps are not vented. I put a couple gallons of fuel in the tank yesterday and ran the engine for about an hour. Noticed the top sucked in this morning.
 

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Your OEM gas cap is supposed to have a vent hole to prevent that. If you're using an aftermarket one, check to see if it's vented. If not, you can drill a small hole to allow a vent or you may have to buy a vented cap.
 

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I run an Aeromotive Stealth II EFI tank with the factory "vented" cap on my '66 and have no problems and no need for an additional vent.
 
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Same as above.
 

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1966 Mustang Hardtop 289 4 Speed
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Discussion Starter #6
The chrome itself is in good shape, the cap looks like it could use a good cleaning. Anyone have any success taking one apart? If all else fails I'll have to buy a replacement.
 

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The vent is a little dimple in the sealing surface of the filler:


My Cougar doesn't have a flip cap but maybe a Mustang cap has something similar;


The inside of my cap has cork and a wavy brass piece. I'm not sure what your car has, but maybe it's full of gunk or something blocking the vent.
 

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1966 Mustang Hardtop 289 4 Speed
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Discussion Starter #8
I can see a hole in the back side of my original cap but it looks to be plugged up. The cable is broken and they are not designed to come apart. I'll pick up another one in my next parts order.
 

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The '66 vented gas cap should look like this. The small hole in the center is the vent.
747781
 
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