1968 Plymouth GTX, 440. Lost my
original gas cap and need to replace it. Is
it a vented or non vented cap? I do know
I have a return line and a overflow hose
that leaks gas after I fill up and drive.
After a few miles. I release the pressure
in the tank by opening up the cap and
the over flow stops leaking. Back to my
original question, Do I replace with a
vented or not vented gas cap?

The cap is not vented. The vent line is
separate, a long U-shaped, highmounted
steel hardline. If releasing the cap results
in audible pressure escape, the vent
line is clogged or somehow screwed
up. Many newer cars used a pressurevacuum
cap which solves problems
of pressure venting and make-up air
allowance, but I would not recommend
this with your near-horizonal filler tube.

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