What is wet stacking and what's the primary cause of it?

Wet stacking refers to the accumulation of unburned fuel and excessive oil in the exhaust system, which occurs primarily when an engine operates at low loads or idles for extended periods. This phenomenon usually leads to fuel and oil pooling in the exhaust pipe, creating a buildup that results in black gooey deposits, often referred to as "combustion slobber."

The primary cause of wet stacking is often prolonged engine idle, which does not allow the engine to reach optimal operating temperatures. During this state, the fuel does not burn completely, leading to excess fuel being expelled into the exhaust system instead of being burned within the combustion chamber. As a result, the unspent fuel and moisture can condense within the exhaust system, contributing to the formation of undesirable deposits.

In an engine that is working within its designed load parameters, temperatures are maintained sufficiently high for complete combustion, minimizing the risk of wet stacking. Thus, extended periods of idling or operating at low power settings are critical conditions that foster this issue.