I don't think it is oil you are burning to produce the black smoke, black smoke is due to overfueling, and the way you discribe it sounds as though the coolant temp sensor has failed.
Swapping parts is all very well and good (what items were swapped would be useful) but has the car been put on a diagnostic computer of any sort?
You should expect to see temp: shown by c/temp/sensor to match the in car gauge roughly, but I believe if put on examiner or even a generic diag: computer yours could be easily showing a minus figure (we have seen -55) therefore c/temp/sensor telling ECU its bloody freezing, so chuck loads of fuel in to get the car started, 'trouble is the sensor isn't going to change its view on how cold it thinks it is even when the car is hot, and it's going to choke itself, literally.
I would be interested to know the outcome,
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I spent ages and ages reading the post thinking and then thinking some more and im very much aware that blue smokes oil, black smoke overfuelling, white smoke water.
ive recently come accross simular symptoms which did reveal to be the oil rings and it emitted black smoke chucked me on a rampage simular to what your mechanics gone through with changing your sensors. has i always casted the black smoke view to be overfuelling.
I also work along side a close friend who works on marine engines hes pretty clued up with all kind of engines so we sat and got our heads together scratched the hell out of them in the process and heres what we revealed.
we was working on an alfa it was the 1.8 156 not the 2.0
we changed everything using a donor car same scenario we swapped over every sensor we could find and still it was the same.
we checked at first the standard parts the plugs fuel pump etc we found no: 3 cylinder spark plug abit coaked in oil not massively only a bit so we cleaned it and thought nothing more about it. this was before we changed the sensors!
Then we decided to check the spark plug gaps and came to removing cylinder 3 plug and again was covered in oil cleaned it put it back started the car checked it again was covered again black smoke been emitted. was thrown off with why the black smoke was been emitted has i stated i always casted it to be overfuelling. so we knew we was up againts the oil rings on cylinder 3 so we started taking the engine to bits replaced the rings started her up and she was fixed no smoke purred lovely and was all caused by cheap oil not been changed when it should have and it left cylinder 3 with abit of scoring on its bore luck has it its running ok still.
so from that its upto you the question was for ideas or simular situations to be posted this is just mine and alfas been alfas you just dont know they have that much computiresed crap it makes it hard to diagnose without alfadiag or examiner to hand.
and from a team of engineers is a website which ill find tonight whos also now placed "Black smoke oil/overfuelling" on there site due to simular finds like myself so it leaves doubt. Diesel engines its easy to say petrols shouldnt emmit black smoke and once you find out why you will resolve your idle non starting missfiring problems the black smoke is your clue start there.