Wet stacking is a condition that can occur to all diesel engines and not just generators. Wet stacking means that not all the fuel is burned and the unburnt residue goes into the exhaust side of the turbo and into the exhaust system. The main cause for diesel generators is that the engine is running at a low proportion of its total capacity.
The symptoms of wet stacking are generally a black ooze around exhaust pipe connections and around the turbo as well as continuous black exhaust from the stack when under a constant load.
A diesel engine has to have exactly the right air to fuel ratio and be able to sustain the operational temperature it was designed to run at for a complete burn of the fuel. When a diesel generator only has light loads, its not able to reach the correct operating temperature, which means not all the fuel is combusted. This leads to wet stacking.
Over time permanent damage can occur as the deposits erode the engine surfaces. Additionally with the piston rings not getting to their designed temperature, unburned fuel and gases can escape into the oil pan diluting the lubricating properties of the oil, leading to premature engine wear.
The best way to avoid wet stacking is to ensure that the generator is run on load with at least 75% of its maximum load capacity so that it reaches its optimal running temperature. This can either be achieved by doing an on load test using the building load or using an external load bank. If wet stacking has already occurred but it is at its early stages, carrying out the same should burn off the unburnt fuel and solve the problem. If wet stacking has occurred for some time, it might be necessary to carry out an engine rehaul to resolve.