"Wet Stacking" occurs when a diesel engine operates below its rated output. The engine begins to over-fuel or "wet stack" if there is an insufficient load to bring the engine up to operating temperature. Diesel engines are designed to operate with a load and consistently operate more efficiently in the 70 to 80% range of rated output. When a diesel engine generator operates for an extended period of time, say below 40%, over fueling occurs.
Wet-Stacking, if allowed to go on too long, can cause permanent damage should the generator be called to perform at or near peak for an extended length of time. This potential catastrophic problem can be avoided if an additional load is applied on a regular basis.
So, what are the causes?
In order for a diesel engine to operate at maximum efficiency, it has to have exactly the right air/fuel ratio and be able to sustain its designed operational temperature to burn the fuel completely. When a diesel engine is operated on light loads, it will not attain its correct operating temperature. At this point unburned fuel is exhausted and noticed as wetness in the exhaust system, hence the phrase wet stacking.
Effects on the engine:
When unburned fuel is exhausted out of the combustion chamber, it starts to build up in the exhaust side of the engine, resulting in fouled injectors and a buildup of carbon on the exhaust valves, turbo charger and exhaust. 
Excessive deposits can result in a loss of engine performance as gases bypass the valve seats, exhaust buildup produces back pressure, and deposits on the turbo blades reduce the efficiency of the turbo.
Permanent damage is not typically incurred over short periods, but over longer periods, deposits will scar and erode key engine surfaces.
In addition, when a diesel engine runs below the designed operational temperature, the piston rings do not adequately seal the space between the pistons and the cylinder walls. This results in unburned fuel and gases which dilutes the oil, leading to engine wear, prematurely.
Things to Consider:
In addition to the adverse engine effect, the user of a system should consider:
- Power - Even before an engine is damaged, deposits reduce the power output. A prematurely worn engine will have a lower maximum power than it was designed to produce.
- Pollution - Many urban areas restrict the level of smoke emissions wet stacking produces.
- Maintenance - An engine experiencing wet stacking will require considerable more maintenance than an engine that is sufficiently loaded.
- Expense - Excessive wet stacking will shorten engine life by many years and before planned replacement.
Wet Stacking Solution:
The obvious solution is to run the generator set under load so that it brings the operating temperature of the engine up to the designed parameters. Built-up fuel deposits and carbon is removed by running the diesel engine at the required operational temperature for several hours.
These days many standby generator system owners outsource complete maintenance of the system and have a planned maintenance (PM) contract with a full service generator-set supplier. During your regularly scheduled planned maintenance visit, they can bring in a portable load bank to run the generator at a load that maintains the designed operational temperature, so that your generator system worries (and wet stacking issues) are alleviated.