I clean them with hot water and mean green cleaner, aided with a parts brush.

The stuff can remove varnished oil like no-ones business.

You can also buy a bucket of carby clean and use that to clean the cover. I have also used mopar combustion chamber cleaner on some parts, It can lift hard baked carbon off if the item is hot.

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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)