I recently bought a new gas cap (when I bought it, it had an incorrect locking cap with no key so I yanked it off) from Spector, part #168-01C for my 72 FJ40. Now, when I start it up, about 5 seconds later I hear the gas tank make sort of an oil can noise. When I shut it off, 5-10 seconds later I hear it again. Turns out the cap isn't venting, and is causing a suction. If I leave it running, and pull the cap off, the idle drops just a hair, and when I put it back on and the vacuum returns, the idle raises just a few rpms I'm assuming because it leans out a bit. These gas caps ARE supposed to vent, right?