Does leaving it idling for a long time harm a diesel engine?
Yes. Running a diesel engine continuously at idle or at very low load does not bring the cylinders up to sufficient temperature; unburned fuel accumulates on the exhaust side. This condition is called wet stacking. The results are glazing on the cylinder wall, thinning of the oil with fuel, soot on injectors and valves, an increase in exhaust back pressure and loss of power.
Wet stacking is not a maintenance error but a question of how the engine is operated. A diesel is a high-compression engine and requires a certain load and temperature for complete combustion. At idle that condition is not met.
Symptoms: dark deposits and seepage from the exhaust, increasing smoke, drop in power, rise in oil level (fuel mixing in).
Practical measures:
- During long waits, stop the engine if possible instead of keeping it at idle.
- Run it regularly at reasonable load; do not just warm it up unloaded and shut it down.
- Change the oil within the manufacturer's change interval; do not leave fuel-thinned oil in place.
The contribution of electronic throttle control here is that it makes the transition between work speed and idle easier: on the GS740 you can drop to idle with a short button press and return to the same work speed. That way the habit of keeping the engine at high speed "so the setting doesn't get lost" disappears.
Full topic: Electronic speed and throttle control on diesel engines