-=- originally posted by Bobelmore
Although I know nothing for sure, I have a few guesses:
1. Should not affect anything while driving a steady speed, like hwy driving.
2. When you take your foot off the pedal & at idle, I believe outside air is drawn into the manifold, the 02 Sensor then instructs the gas mix to become more rich than necessary. This races the engine rpm slightly & perhaps enough to keep outside air from entering the manifold, then the rpms will fluctuate as the 02 Sensor goes a bit crazy with the mixture.
3. Guess #2 will cause the 02 Sensor to throw a 21 code, especially when releasing the throttle after hwy driving.
4. Outside air gets into the manufold overnight, causes the )2 Sensor to race the engine (1100-1400rpm) for a couple minutes.
5. Since exhaust mostly still exits the rear, no need for Carbon Monoxide concerns.
6. So leaking manifold is really only a noise issue.
Yup, I was bored enough to write this.