Thick black smoke means a blown gasket, probably.
When a gasket blows up, oil gets into the combustion chamber and the smoke you're seeing is the half burnt oil.
And when that happens, spark plugs get covered with carbon and hence can't work at their optimum level hence the sudden hike in the fuel consumption.
So, replace the gaskets, spark plugs, either top up or completely replace the engine oil, check compression level of each cylinder, check the catalytic converter for any blockages, check the EFI system and hopefully your car will start running great again.
There's no easy fix or shall I say جگار for your problem so expect to pay north of 20k.
Best of luck.
