Охладитель EGR забит сажей

The cooler's internal passages blocked with soot, so recirculated gas is neither cooled nor flowing freely. The cooler is a small heat exchanger with narrow gas channels — exactly the geometry that clogs fastest — and it sits at the dirtiest point in the system.

It fails in two ways that present quite differently. Blocked, it restricts flow and delivers hot gas, which shows as low EGR flow, a temperature correlation fault between the sensors either side of it, or raised nitrogen oxide output.

Split internally, it does something far more serious. The gas channels run through engine coolant, so a crack lets coolant into the intake — which shows as white exhaust smoke, coolant disappearing with no visible leak, and eventually hydraulic damage if enough collects in a cylinder. Unexplained coolant loss on a diesel with EGR faults is worth taking seriously as this.

Cleaning is possible but less rewarding than on a valve, because the channels are narrow and the deposits are baked. Many are replaced.

As with everything in this family, short cold journeys are the underlying cause — a cooler that never gets hot accumulates what a hot one would burn off.