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What Is Wet Stacking and How Do You Prevent It?

What Is Wet Stacking and How Do You Prevent It?

A man with a neon vest looking out at two yellow excavators on a construction site

What Is Wet Stacking and How Do You Prevent It?

Reading time: 5 min

A lesser-known, but critical condition that can undermine the efficiency and longevity of your diesel equipment is “wet stacking.” Wet stacking occurs when unburned fuel, along with oil and carbon deposits, accumulate in the exhaust system of a diesel engine. This usually happens due to the engine operating below its optimal temperature range, often at low load levels for extended periods.

To prevent wet stacking, it’s important to operate your equipment correctly and take the necessary maintenance precautions it requires. 

Reducing the amount of time your equipment spends idling at low temperatures and implementing a thorough maintenance plan is crucial to prevent wet stacking and reduce its impact on your equipment.

In this guide, we will cover the nuances of wet stacking and how it can be prevented. By adopting a proactive approach to maintenance, operators can enhance efficiency, reduce the environmental impact and extend the service life of their diesel equipment.

Wet Stacking In-Depth

Wet stacking occurs in diesel engines under certain operational conditions, especially when the engine is consistently run at lower loads than its designed capacity. This underutilization prevents the engine from reaching its optimal operating temperature, leading to the incomplete combustion of diesel fuel. 

The incomplete combustion allows unburned fuel, along with oil and carbon residues, to accumulate in the exhaust system, manifesting as a greasy, dark discharge hence the term "wet stacking."

Heavy diesel equipment like skid steers, dozers, excavators and generators are particularly susceptible to wet stacking. These types of machinery are designed for heavy loads and intense labor. When they are used for tasks well below their capacity or left to idle for extended periods, however, they run cool and wet stacking can occur. This is especially common in situations where equipment is used for light-duty tasks that fail to engage the engine's full potential.

Signs of Wet Stacking

Identifying wet stacking early is crucial to reduce its impact on your machine. Signs of wet stacking occurring include noticeable emissions of dark, oily exhaust, a reduction in engine power and efficiency and unusually high fuel consumption

Recognizing these signs allows operators to address wet stacking before it leads to more serious problems.

What Causes Wet Stacking?

Wet stacking in heavy equipment could be caused by any number of operational and mechanical issues. Here are the most common causes that lead to wet stacking in heavy equipment.

Low Load Operations

Operating below the machine’s optimal capacity leads to incomplete combustion, causing unburned fuel and residues to accumulate in the exhaust.

Prolonged Idling

Extended idling keeps the engine running at a suboptimal level, preventing it from reaching the necessary temperature for complete combustion and contributing to wet stacking.

Incorrect Fuel-to-Air Ratio

An imbalance, often due to fuel injector or air intake issues, results in incomplete fuel combustion and the buildup of unburned residues.

Inadequate Engine Temperature

Proper combustion requires an optimal engine temperature. Temperatures too low or too high hinder efficient fuel burning, creating the conditions for wet stacking to occur.

The Impacts of Wet Stacking on Diesel Equipment

Wet stacking can have significant effects on the performance of your machine’s engine. Over time, the accumulation of unburned fuel and carbon deposits leads to a reduction in the engine’s power output. This inefficiency can impact performance and the engine may begin to struggle to meet the demands of heavy-duty tasks.

The impact of wet stacking extends into other engine components, as well. For example, fuel injectors can become clogged or damaged due to the unburned fuel residues. 

Exhaust components, including the turbocharger and exhaust manifold are also at risk. The buildup can lead to corrosion and physical obstruction, diminishing the exhaust system's functionality and leading to costly repairs.

Additionally, wet stacking leads to increased fuel consumption. The engine's compromised state requires more fuel to be burned to achieve the same power output, increasing operational costs.

Environmental concerns are another significant aspect of wet stacking. The incomplete combustion process results in higher emissions of pollutants, including unburned hydrocarbons, particulate matter and carbon monoxide. These emissions contribute to air pollution and can lead to the equipment failing emissions tests.


Wet stacking is best prevented by implementing certain maintenance and operational best practices. The prevention measures below can help reduce the likelihood of wet stacking and mitigate its effects.

Optimal Load Operations

Ensure that your equipment’s diesel engine operates within its designed load range as much as possible. Avoid prolonged periods of low-load operations or excessive idling. 

Utilizing the equipment for tasks that engage its full capacity helps maintain the engine's optimal temperature for combustion, preventing the buildup associated with wet stacking.

Fuel Quality

Use high-quality diesel fuel with the appropriate cetane ratings to ensure complete combustion. Lower-quality fuels can contain contaminants that may contribute to incomplete burning and the accumulation of residues. Regularly checking and managing fuel quality can go a long way in preventing wet stacking.

Maintenance Schedules

It’s critical that you adhere to the maintenance schedule recommended in your machine’s service manual. This includes regular inspections and cleaning the fuel injectors, air filters and exhaust systems. 

Ensuring that these components are in good working condition helps maintain the correct fuel-to-air ratio and optimal combustion temperatures, reducing the incidence of wet stacking.

Engine Modifications

In some cases, retrofitting older engines with modern combustion technologies or exhaust treatment systems can improve efficiency and reduce emissions. These modifications can help the engine burn fuel more completely, helping to prevent the conditions that lead to wet stacking.

Final Thoughts

Preventing wet stacking is crucial for maintaining the health of your equipment’s diesel engine. Wet stacking not only harms engine performance but can also lead to significant damage of components like the fuel injectors and exhaust systems. 

If you’re looking for high-quality components to address or prevent wet stacking, the EquipmentShare Shop offers a comprehensive range of OEM and aftermarket engine parts.

Whether you're looking to replace injectors, exhaust system parts or other engine components, ensuring your equipment is fitted with the best parts available is a crucial step in preventing the harmful effects of wet stacking. Don’t see the specific part your equipment needs? Contact one of our parts experts and get personalized assistance.

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