Black smoke and power loss under load on a Mercedes GLS X167 diesel usually point to an air/fuel imbalance, often linked to EGR, DPF, or boost issues.
Have the codes and live data checked before replacing parts, and avoid hard driving if the smoke is heavy or the car is in limp mode.
What black smoke and power loss usually mean on a GLS X167 diesel
On a Mercedes-Benz GLS X167 diesel—often the OM656 inline-six in 350d/400d variants—black smoke under load is typically a sign of too much fuel for the air available, or not enough clean air reaching the cylinders. That can come from a restricted intake, a boost leak, a sticking turbo VNT mechanism, a clogged EGR valve/cooler, heavy intake-manifold carbon, or a DPF that is loading up and raising exhaust backpressure. A clogged DPF by itself more often shows up as power loss and regeneration trouble, but it can overlap with EGR and boost faults. It is not a one-fault-fits-all situation, so the best first step is to read fault codes with Mercedes Xentry or a capable scan tool and look at live data before replacing parts.
