With the mentioned loads, it is not surprising that glow plugs fail. They should provide at least 15,000 engine starts, but often the plug fails earlier. Sometimes it even happens that the tops of the glow plugs in the engine simply burn out. Burnt glow plugs (the pins of the plugs look "gnawed off" in this case) are mostly the result of faults in the injectors. In this case, the injectors should be checked for serviceability or replaced if necessary.
A shifted fuel delivery timing from the high-pressure fuel pump (HPFP) can also significantly reduce the service life of glow plugs.
Removal glow plugs
1. Disconnect the ground wire from the battery. This is necessary because otherwise the connecting wires will be under voltage when the ignition is on.
2. Unscrew the spark plug wires of the two cylinders located on the left in the direction of movement.
3. Unscrew the wires from the glow plugs.
4. Unscrew the glow plugs using a SW 12 wrench.
5. When installing, tighten the glow plugs to 25-30 Nm.