Would exhaust leak contribut to rough idle?

[Mless5]

I need to replace gaskets around the cat, meanwhile it got me thinking. Does it affect idle speed? It's not visible on the tachometer, but I can hear fluctuation (sp?).

Thanks!

[rlomba8204]

Hard to say. In general, however, poor exhaust flow, or restricted flow, such as that caused by when your car's replacement cat is too small and not OE, will cause the pistons to run a tad hotter and can contribute to driveability issues.

[Brad D.]

I don't recall offhand what Motronic 1.0's o2 sensors strategy is idle ( whether its open or closed loop) as Im running Motronic 1.3. However if its closed loop at idle, a leak in front of the cat/02 sensor could definitely cause a problem as the ecu will see a lean condition and richen the mixture which could definitely affect your idle.
I would also address the other classic causes of a bad idle on these cars and clean/replace the IACV and make sure that your TPS/idle switch is adjusted properly. Also make sure you are running good OE Bosch plugs (WR9LS IIRC) and gap them correctly.

[Mike W.]

It could definitely affect idle. Exhaust is a pulsing and if you have a opening (leak) before the O2 sensor, it will push some exhaust out and pull some air in. Thereby throwing off the true O2 sensor readings. And with all O2 equiped BMW's I've had, going back to L-jet with lambda and K-jet with lambda, they run closed loop at idle. They may take a little longer to get to closed loop without a heater, but regardless, they do run that way.