Thank you everyone, my spare cap (used) is not vented, the tank is not ESC type, it has a vent with a little cotton swab in it, car starves for gas at about 2600 rpm and 55mph is top speed in fourth gear beyond that it backfires and stumbles, this began when I put spare cap on...hope problem is solved with proper cap, stolen cap was marked "vented" I assume it was proper, thank you rivnut, ordered from buickparts, hopefully all will be well again, thank you all again, Pat