Mercedes-Benz E250 CDI smoking and smelling while idling could be caused by many things, but the most common reasons are a faulty fuel injector, a clogged air filter, a worn-out piston ring, or a damaged cylinder. If the smoke is blue, it indicates that oil is burning and if it smells like burning, it is a sign of a mechanical problem. Blue smoke could be caused by oil leaking into the engine's combustion chamber, resulting in a problem with the engine oil system. The clogged air filter restricts the amount of air reaching the engine, causing it to run rich, which could lead to burning fuel, giving off black smoke. A worn-out piston ring or a damaged cylinder prevents the engine from properly compressing the mixture of air and fuel, resulting in burning and smoking. In any of these situations, it is advisable to take your car to a certified mechanic as soon as possible to diagnose and fix the problem.