<!-- from fixya rte -->Elantra 2007 crdi heavy black smoke - Elantra Hyundai Cars & Trucks

check for a turbo and if it has one , have it replaced

Would check air filtre and fuel filtres firs t

has this engine had a re-ring/rebuild lately
how many miles has it done
has it been overheated at any time lately
have you run the fault codes at all
to get some sort of reasonable answer to your statement then re-ask the question and include all the information

Bad coolant temp sensor. This part takes the place of old style choke, and when bad ECM doesn't know engine has warmed up so fuel mix stays too rich.

the cause of the heavy black smoke is un-burnt fuel. is from excessive fuel pressure, faulty injectors,ignition timing out. If it is a turbo operation then the turbo is not producing the correct boost for the fuel settings . Check turbo for vane actuation . low power comes from overfuelling, low air flow. run the fault codes to see which sensors are out of parameter.

Check the glow plugs but it sounds like the injectors and pump needs to be serviced.

It almost sounds as if the rev limiter is kicking in.

Hi there:Black smoke is caused by excess fuel that has entered the cylinder area and cannot be burned completely. Another term for excess fuel is "running rich." Poor fuel mileage is also a common complaint when black smoke comes out of the tailpipe. Black smoke out the tailpipe is the least cause for alarm. Excess fuel will usually effect engine performance, reduce fuel economy, and produce a fuel odor.
How did the fuel get into the cylinder in the first place? Some of the causes of excess fuel are a carburetor that is out of adjustment, a faulty fuel pump, a leaky fuel injector, or a faulty engine computer or computer sensor. If black smoke is present, check the engine oil as in the white smoke example to make sure excess fuel has not contaminated it. Do not start the engine if a heavy, raw fuel smell can be detected in the engine oil. Call your mechanic and advise him of what you have found.
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Black smoke is from too much fuel. Possible coolant sensor problem??? You can have this tested quickly with a live data scanner from a mechanic.