89 iroc it came with a vented aftermarket murray brand gas cap. I am having trouble passing emissions through the tailpipe and I was thinking putting the stock type non-vented gas cap may help (?) but the aftermarket non-vented caps won't fit. It goes on but won't go all the way down and seal before clicking. I am thinking the gas tank may have been replaced by a reproduction one at one time though. Maybe that is why the non vented one doesn't fit?

 

Had this issue with the factory stock gas cap and tank. Was worse on hot days, so bad that the cap would hold the door open some.

To fix it spent a minute or so with some files on the gas cap. Used a round file (rat-tail) to deepen the grooves a little. Then a flat file over the tops of the peaks of the threads. Doesn't take much being plastic and all.

RBob.