can fouled plugs cause high BLMs?

Originally posted by TurboDave
Can't imagine how.

Yes. Fouled bad enough not to be firing all the time leaves raw gas in the pipe. That doesn't count as in that doesn't generate any O2, so the O2 sensor see lean, and then the ecm commands a rich correction.
I would have to disagree. Seems to me that unburned fuel in the exhaust stream would be sensed as a rich condition.
Am I wrong?
At idle it would certainly cool the O2 sensor (same as a rich conditon).
Since the O2 only senses oxygen, if the fuel is not fired, then fuel and air would be pushed out of the cylinder (with the fouled plug), the O2 will sense O2 (from the air) and try to drive the mixture richer, to no avail. So, Bruce's assessment is correct.