What is a typical indicator of wet stacking in diesel engines?

A typical indicator of wet stacking in diesel engines is the presence of oily residue in the exhaust system. Wet stacking occurs when excess fuel is injected into the engine during low-load operations, leading to incomplete combustion. This unburned fuel can mix with lubricating oil and result in a carbon buildup in the exhaust system. The accumulation of this oily residue is a sign that the engine is not operating efficiently, typically due to prolonged idling or operating at low RPMs. Identifying oily residue can help in diagnosing the issue and taking corrective actions, such as adjusting the load conditions or maintenance procedures to improve engine performance.

The other options, while they may indicate other issues with diesel engines, do not specifically relate to wet stacking. For instance, black smoke often indicates a rich fuel mixture or incomplete combustion but doesn't necessarily confirm wet stacking. Excessive noise levels could point to mechanical issues rather than combustion inefficiencies. High engine temperatures generally suggest an overheating condition, which is not directly linked to wet stacking. Therefore, the presence of oily residue is the most telling sign of this specific condition in diesel engines.