I don't know the spec off hand but the primary side should be less than 1ohm. Testing on the 20k scale is useless.

The proper way to test a coil is dynamically with an oscilliscope. Looking for available voltage and proper oscillations.

I seriously doubt that your rough idle is caused by the coil, but I suppose anything is possible. Idling is one of the easiest situations for the coil. Starting and acceleration tax a coil much more.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician