What is wet stacking?

Wet stacking is a condition that can affect all diesel engines, 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.

Main Causes

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 awet stackinground exhaust pipe connections and around the turbo, along with continuous black exhaust from the stack when under a constant load.

A diesel engine requires exactly the right air to fuel ratio,  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, it’s not able to reach the correct operating temperature. meaning not all the fuel is combusted. This inturn leads to wet stacking.

Over time permanent damage can occur as the deposits erode the engine surfaces. Additionally, the piston rings do not get to their designed temperature. Causing unburned fuel and gases to escape into the oil pan. This dilutes the lubricating properties of the oil. Thereby leading to premature engine wear.

Run the generator on load to avoid wet stacking. With at least 75% of  maximum load capacity. Reach optimal running temperature. Achieved this 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.

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