What color is it? If blue it could be several reasons.
Clogged air filter element (Replace air filter element)
Obstructed air intake to the turbocharger compressor (Remove obstruction - repair parts as required per engine manual.)
Intake system air leakage (Correct leak by tightening the fasteners or replacing gaskets)
Obstruction in exhaust manifold (Tighten fasteners or replace gaskets as required.)
Blocked oil drain line (Remove obstruction or replace oil line as required)
Crankcase not vented (Clean breathing system per engine manufacturer's instructions.)
Turbo bearing housing coked, carbonated or sludged (Repair or replace the turbo and change engine oil and filter)
Internal engine problem (Repair worn valves, liners, pistons, piston rings, etc, as required.)
Black...
Clogged Air filter element Replace air filter element
Obstructed air intake to the turbo compressor (Remove obstruction-repair parts as required.)
Obstruction in the intake manifold system (Remove obstruction per engine manufacturer's instruction.)
Intake system air leakage (Correct leak by tightening fasteners or replacing gaskets.)
Obstruction in the exhaust system (Remove or repair obstruction per engine manufacturer's manual)
Exhaust system gas leak (Tighten or replace gaskets on surfaces between turbo/manifold/engine.)
Fuel injection pump or injectors out of calibration (Replace or adjust fuel system as required)
Internal engine problem (Repair defective camshaft, valves, liners, pistons, piston rings etc, as required.)
Material build up on the compressor wheel. (Clean with non-caustic cleaner. Find and correct dirty air source.)
