Had an odd occurance I'm wondering if anyone else has experienced. About a month ago I had to add gas to my wagon. Undid the gas cap and had a rather strong expulsion of air/vapors/spittle from the filler neck. Thought that was odd, but since I had run it down to almost empty, thought it might just be due to there being so little gas in the tank (running on fumes). But, I continued to have this experience for a number of fill-ups, maybe about 3-4. Then, I thought to myself, I'll check it once or twice a day whether I've run the car or not and see if it seems to stay pressurized. Of course then it stopped having this condition. And now for the last couple weeks, there has been no observed abnormal pressure when I remove the gas cap.

Time for a new gas cap or what?