Under the loads mentioned, it is not surprising that glow plugs fail. They should provide at least 15,000 engine starts, but still often the spark plug fails earlier. Sometimes it even happens that the tops of the glow plugs in the engine simply burn out. Burnt glow plugs (candle pins look like this in this case «gnawed») are mostly the result of malfunctions in the injectors. In this case, the injectors should be checked for serviceability or replaced if necessary.
A shifted timing of fuel supply from the high pressure fuel pump can also significantly reduce the life of glow plugs (injection pump).
Removing glow plugs
1. Disconnect a wire of weight from the storage battery. This is necessary because otherwise, when the ignition is on, the connecting wires will be energized.
2. Turn off wires of candles of two cylinders located at the left in the direction of the 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.