One of the worst designed airboxes in my opinion is the Td5 defender and Disco. I regularly have to use rubber grease around the sealing surface to ensure a decent seal after pulling the old one and seeing the leakage.
Dusting a td5 isn't as easy as it sounds, the oil misted intercooler also traps a fair bit of grit before it gets to the inlet manifold, where again it is adhered to the slop from the EGR valve...
IME dusting a Td5 is the result of a LONG time of poor maintenance, not so much a few hundred km of dirt roads with a clogged/ leaking filter element. Almost every td5 I service requires a MAF clean due to dust and oil mist from the breather system, and YES the crankcase fumes DO travel upstream after the engine is switched off, and will leave a residue in the MAF ready to attract the next lot of dust entering the airbox....at least this appears to be the case IME.
Inspecting the turbo inlet housing is even more an indicator, I have seen some amazing abrading done to the compressor vanes over the years from dust ingress 
JC
The Isuzu 110. Solid and as dependable as a rock, coming soon with auto box😊
The Range Rover L322 4.4.TTDV8 ....probably won't bother with the remap..😈
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