Depending on the amount of false air being introduced, so would symptoms vary. It is as though you had a carburetor and were adjusting the idle mixture screw. Also, the colder/damper the air, the more oxygen per cubic foot being unaccounted for. It would take less than half an hour to rule 90% of intake related vacume leaks out. An 02 sesnsor does not tune per cylinder rather on overall output therefore you would also likely have the other cylinders running too rich. This would all but disappear after the throttle was opened due to the proportionaly minute amount of false air. On the 129's for instance, a plug reading is almost worthless as they run so lean at idle. I have chased lean stumbles on these cars many times that will exhibit these very symptoms. It is definately worth checking on before anything else. Steve

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64 European Coupe Brilliant Silver with Red interior 4 Speed Sunroof
87 TDT 286k
87 SDL 195k
83 SD 202k
83 SD 201k
83 D 286k
02 R1150RT 32K
00 122HP FLSTF
07 KTM 250SX

"Danger takes many forms"