benedict explained it perfectly

your case maybe due to damaged wiring too, this year had the biodegradable wiring in it whose insulation crumbles away. Reason for me say this is when I bought my car its idle control was dead, meaning everything worked except the idle. Very lumpy and harsh.

my case was a misconfigured connector for the ELR cube. Once I fixed it, the idle is extremely smooth no matter if the engine is started dead cold, or I switch on other things like A/C or defrost etc even on a cold engine.

The OM606 engine whilst idling makes a clicking noise only - its very quiet and almost mouse quiet when the hood is shut closed and you have its barrier blanket pasted on the underside.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)