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My 70 428CJ stalls after about 20-30 minutes of driving. I opened the pop open gas cap and I heard the telltale "whoosh." The problem is, I can't see any vent.

The car spent many years in California before I bought it, although its DSO is Omaha. I guess it's posible the PO replaced the original cap with a non-vented cap. I tried a replacement twist-on cap and the problem went away. The gas tank is not vented.

Here's the question: Can I vent the cap myself by drilling a hole in the center button, like the twist-on cap has, or is the pop-open cap vented differently?

Thanks,
Joe
 

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Never really took a good look at a pop open but it should vent through the edge of it like the normal Mustang caps.On the inside of the cap where it tightend against the tank inlet there should be a cork gasket.A small hole should be inthe inside edge of the cap and it should vent out under the outside edge ..Check the inside of that cap and see if there is a hole that is plugged up.It has to have a vent. The unvented tanks have a small hose running up from the tank and out into a cannister of on some just out the top of the fill line.
Got me now, I will have to check on these other caps and see where it is vented to..
 
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