Similar problems
I had similar problems. It was rusted area leaking on flex pipe. It only got inside car when I was circulating outside air and at idle speeds so the CO2 could build up enough to get into the intake. I set my intake to recycle inside air until I could get the leak fixed.
I have a 95 Mazda Protge. It has been a bit rough when decelerating and has been sputtering and grumbling when I let up on the gas to decelerate lately. yesterday i started it and it started up fine then after a few seconds it stalled out. It has idled rough for some time now so I restarted it...