What part of the diesel engine is defective when muffler blow bluish smoke?

Bluish-white or blue smoke from a diesel engine's exhaust usually indicates that engine oil is being burned. This points to a problem with the engine's internal components, not necessarily the muffler itself. The muffler is simply the exhaust's silencing device; it doesn't cause the oil burning.

The specific cause could be several things, including:

* Worn piston rings: These prevent oil from entering the combustion chamber. If they're worn, oil leaks past, gets burned, and produces blue smoke.

* Worn valve stem seals: Similar to piston rings, these seals prevent oil from leaking into the cylinders past the valves.

* Damaged cylinder head gasket: A leak in this gasket could allow coolant or oil to enter the combustion chamber. If it's oil, you'll see blue smoke.

* Excessive turbocharger wear: A failing turbo can leak oil into the intake, leading to combustion and blue smoke.

The muffler itself is not directly causing the problem; it's simply carrying the exhaust fumes that contain the burned oil. Diagnosing the exact cause requires a mechanic's inspection.