October 26, 2022

Wet Stacking: Causes, Effects, and Prevention


Wet stacking is a major problem that can arise if your diesel engine is not properly maintained. You might be forced to replace your generator before its time, making you incur a high cost. Luckily, we're bringing you tips on preventive maintenance against wet stacking.

In this article, we'll discuss what causes wet stacking and its effects on your generator. We'll also take a look at signs that indicate your generator is undergoing wet stacking. Let's begin. 

What is Wet Stacking?

Wet stacking occurs when a diesel engine accumulates a thick and dark liquid in its exhaust. It's mainly caused when unburnt fuel mixes with moisture and carbon particles to form a muddy liquid.

You can detect wet stacking by looking out for these signs:

  1. Presence of a dark ooze dripping from the exhaust pipe.
  2. Emission of black exhaust from the stack when it's under steady load.

Causes of Wet Stacking

The leading causes of wet stacking in diesel generators are:

  • Operating your generator below its designated temperatures
  • Using an inaccurate air-to-fuel ratio in your generator
  • Running the engine at less than 60% power load
  • Using little or too much fuel
  • Neglecting your generator for a long time.

How Wet Stacking Causes Engine Wear

Wet stacking can be very detrimental to the performance of your diesel engine. For instance, your engine emits more carbon particles at a lower load percentage than higher.

Research shows that carbon monoxide emissions are two to three times greater at 25 percent engine load than at 75 percent load. 

These carbon particles accumulate on various engine parts and undermine their efficiency. Let's take a look at the affected parts of a diesel generator and how wet stacking leads to the overall wear of the engine. 

The Piston Rings

Once you power down the generator, the unburnt fuel mixes with carbon particles and gets attached to the piston rings.

Due to this accumulation, the piston rings fail to properly expand and seal the space between the pistons and the cylinder walls. As a result, the unburnt gases leak into the oil pan and tamper with the lubricating oil, causing friction and engine wear.

Injector Nozzles

When the carbon deposits clog injector nozzles, the nozzles release larger fuel droplets to the combustion cylinders. The production of more fuel in your engine means that they'll be more unburnt gases leading to more wet stacking.

The Exhaust

The carbon deposits and unburnt fuel cause exhaust build-up, generating back-pressure in the exhaust system. In severe circumstances, stack fires break out as a result.

When wet stacking affects the efficiency of these three parts, it won't take long for the engine to fail permanently.

Preventive Maintenance And Fixing Wet Stacking

Prevention is better than cure, even when maintaining diesel generators. You'll not only prevent pollution from the smoke emissions but also save up on the costs of maintaining an engine with wet stacking.

Research shows that a diesel engine should run at no less than 60% of its rated output power. You can avoid wet stacking by running your generator at least once a week for a couple of minutes.

If you notice wet stacking in your engine, run your generator at 75 percent load for a few hours. The process raises the temperature of the combustion gasses, and the unburnt fuel evaporates and gets blown out as soot.


Failure to control wet stacking might have you incur the cost of a new generator. Fortunately, our experts at Power System Services can help. We specialize in conducting engine overhauls for generators with a lot of carbon buildup. On the other hand, if you're looking to buy one, here are tips for choosing the right generatorContact us today and let our experts assess your needs based on the state of your generator.