Mercedes-Benz W124 diesel blue smoke: causes and head gasket
What causes blue smoke in a Mercedes-Benz W124 diesel?
Blue smoke means oil is burning. The cause can be valve stem seals, worn or stuck piston rings, a leaking head gasket, late injection timing (e.g. a stretched timing chain) or the injectors. Oil consumption is rare in OM601–603 diesels unless the head gasket has failed.
How do I tell that the head gasket of a Mercedes-Benz W124 diesel is leaking?
When the gasket leaks at cylinder number one, oil gets into the cylinder and air gets to the hydraulic lifters. Symptoms are a rattling engine noise, oil consumption and smoke at idle. The repair is replacing the gasket, which involves removing the head and checking its flatness. The head should not be opened before the cause is confirmed.
Why does the head gasket of a Mercedes-Benz W124 diesel leak again after replacement?
The most common cause is reusing old head bolts or wrong tightening torques. Another cause is an old gasket left on the mating surface. More rarely the cause is elsewhere, for example a crack in the head.
Can blue smoke and rough idle be caused by piston rings?
Yes. If a ring is damaged or the grooves are carbonised and the rings are stuck, oil burns and the cylinder wall can be scratched. Then one cylinder does not compress properly and the idle is rough. The repair is honing the cylinders and replacing the piston rings, and the cause must be confirmed before disassembly.