I have had simular problems with my clio. the head gaskets fail. mine too started off by simply over-heating, but the coolant does drop considerably, oil again is a problem as it is leeking from the deck down the side of the gearbox. so its obvious the oil-ways are far too close to the water jacket / outer edge of the head. in your instance i belive you have a leek from the oil-way to the cooling system. the oil is getting collected on the thermostat and keeping it closed. but unfortunately i think its too late for your engine, as black smoke is never a good sign. if you engine still cranks freely start with a compression test, for about £15.00 you can buy a tool to do this, take out the plugs and connect the guage, you should get around 9bar on all 4. if its below 6 on any cylinder your engine is damaged beyond eccanomical repair.again draining the oil and inspecting for anything untoward is worthwhile.
if your lucky and you stil lhave compression, then i would fit a new head gasket (special tool required) flush the coolant system with washing up liquid and fresh oil and filter. the only thing to remember is to set the engine to top dead center usually marked on the crank pulley and avoid moving it or the camshaft during the time you have the cambelt off. you can find Top dead center in other ways, so don`t be dishearted if there is no marks on your pulley. remember the engine rotates clockwise so if there is a few marks then it will be the first marking you come too if you imagine turning the engine.
Finally Good luck.
Dave
August 2007
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