Wet stacking is a condition in diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s in which not all the fuel is burned and passes on into the exhaust side of the turbocharger

Turbocharger

A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

 and on into the exhaust system

Exhaust system

An exhaust system is usually tubing used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes...

. The word "stacking" comes from the term "stack" for exhaust pipe or chimney stack. The oily exhaust pipe is therefore a "wet stack".

In diesel generator

Diesel generator

A diesel generator is the combination of a diesel engine with an electrical generator to generate electrical energy....

s, it is usually because the diesel engine is running at only a small percentage of its capacity.

It is detectable when there is a black ooze around exhaust pipe connections and around the turbocharger. Continuous black exhaust from the stack when under a constant load is also an indication that all the fuel is not being burned.