The copper washer under the injector is a one-time seal: it is permanently crushed when torqued, so once refitted it can no longer seal. If the seal goes, combustion gas escapes past the injector at over 2000 degrees and over 300 bar, carbonises the engine oil and eats into the seat of the injector. The fault appears at around 150 000 kilometres, but an early failure can also be caused by incorrect torque, a dirty seat and a reused washer.

The escaping gas mixes the unburnt hydrocarbons with the engine oil under the rocker cover, and a hard, coal-like crust bakes out of that. This deposit surrounds the injector and over time glues it into the cylinder head.

The symptoms are easy to recognise. At idle a rhythmic chuffing noise is audible, one per combustion event. A diesel smell comes from the engine bay, and through the ventilation into the cabin as well. Black, oily deposits are visible at the base of the injector. The engine oil becomes diluted with fuel, and whitish-blue smoke can appear too.

If things are taken past this point, the carbon literally welds the injector into place, and a dedicated injector puller or a slide hammer is needed to get it out. Prying damages the body of the injector. If the seat has already been eaten away, the cylinder head has to be machined or replaced.

The quality of the repair decides matters here. A new copper washer is needed, the old one can never be reused, the seat has to be cleaned with a hand-turned seat cutter, and the torque has to follow the factory figure. Cleaning the seat is the most important step and the one most often done wrong. If one injector needs replacing, typically all four do, because their state of wear is similar, and a new one alongside worn ones does not give the right metering.