Buy a scan gauge or equivalent.
I cannot stress how valuable they are for the CRD's.
They pay for themselves in fuel savings alone because you can see when you are using the most fuel and even a minor adjustment in driving techniques can save you a significant amount.
Good idea to go back to the Nissan Air Filters, the CRD's are touchy about air flow.
It didn't help you this time but I always used genuine after several forumites encountered issues with afternarket jobbies
Black smoke is burnt fuel (usually overfuelling/MAF/Boost) white is unburnt fuel (usually low compression) and blue is oil so your noticing that the fuel economy has dropped and black smoke is part of the same issue I reckon.
If the dudes who cleaned the MAF damaged it that should show up during voltage tests but the symptoms point to eliminating the MAF as a problem first.
It is also possible they have damaged/squashed/split an air line for the VNT control so triple check all those dinky little hoses on the pax side of the engine.
The big advantage of a Scangauge will be you can see the boost pressure so you can suss out whether boost is low or not.
They don't show EGT but unless you are chipping an engine or doing serious mods you don't need to know EGT anyway.
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