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do diesels like petrol engines have positive crank case ventilation? If they do would be worth checking the valve and the pad inside the aircleaner.

Sound like you might have a little blow-by


What is Blow-by?

Blow-by occurs when the explosion that occurs in your engine's combustion chamber causes fuel, air and moisture to be forced...

Generally I'd lube the new bolts aswell, just in case you didnt know :)