When considering a different generator configuration, supplemental loading or corrective testing, it is important to consider minimum running load as well as peak demand5 and this guide helps you to do so.
What Is Wet Stacking in a Generator?
Wet stacking on the diesel generator is manifested by thick, black exhaust deposits with carbon, unburned fuel, and sometimes even lubricating oil in the manifold, at the turbochargers and at the exhaust pipe joints.1
This may look like an oil leak but don’t assume that the engine is permanently damaged because of exhaust sludge. You need load records and an engine inspection to determine the cause, not an overhaul approved from a photo of the engine, said your team.
This article is about diesel generator wet stacking. Natural gas engines present other low-load issues; shifting fuel does not obviate the need to review the engine’s operating constraints.Different low-load issues exist for natural gas engines; a fuel switch will not eliminate the need to review the engine’s operation constraints.2
When considering a maintenance or replacement decision, consider visible sludge as an opportunity to explore the operating conditions, rather than an automatic conclusion about the engine.
What Causes Wet Stacking in Diesel Generators?
Even if a diesel generator starts reliably, it can be too cool for effective combustion and cause buildup of deposits in the exhaust as a result of extended low-load operations.3
Common situations include:
- Repeated no-load exercise: You confirm that the engine starts, but the test does not demonstrate loaded operation.3
- Low baseline demand: Your generator can support the maximum demand, yet spends long periods carrying only a small part of it.1
- Cold operating conditions: Low ambient or jacket-water temperature can contribute to exhaust slobber.2
Don’t increase the engine speed instead of loading. Exhaust deposits that form at high idle with little or no load are a concern, according to Caterpillar, as your team should use actual electrical kW and temperature records, not engine speed, as the evidence of whether or not combustion conditions are appropriate.2
When comparing PEGASO’s silent diesel generators, published product family provides a basis for selection,7 but this does not mean that the exact engine model, duty rating and written low-load limits have been specified with the purchase.
Specify both your bottom and your top load – a supplier can then see how the proposed generator will perform in practice.
What Are the Signs and Risks of Wet Stacking?
Do not make smoke or soot your diagnosis; dark liquid at exhaust joints is a warning sign, but should be assessed in the context of the load history, temperature readings and service findings.1
| What your team observes | What you should verify | What the finding changes |
|---|---|---|
| Thick, dark liquid at exhaust joints | Applied kW, duration at low load, engine temperatures, and inspection findings | Establish whether underloading is a likely cause before authorizing repairs.1 |
| Smoke during operation or load changes | When it occurs, related alarms, and the technician’s combustion and fuel-system checks | Keep other faults in the investigation; the appearance alone is not conclusive. |
| Reduced power or poor response | Exhaust condition, service history, and loaded performance | Determine whether deposits or component wear require more than an operating-schedule change.1 |
| Aftertreatment alarms, where fitted | The specified exhaust-temperature conditions and diagnostic records | Confirm whether low-load operation is affecting the emissions system.2 |
Low loading times can lead to higher deposits and wear of components. Caterpillar also specifies oil dilution and diminished turbocharger performance as potential issues.1 Aftertreatment equipment requires its own temperature, and clearing exhaust deposits does not prove that all the affected systems are performing correctly.2
To make your repair decision, ask for a diagnosis relating the symptoms to the operating history and determine which components to inspect or service.
How Do You Address Generator Wet Stacking?
The cause should be evaluated and then a burn-off should be done in a controlled manner using the manufacturer’s loading procedure where applicable, instead of doing a full load burn-off on the spot.3
- Record the condition. Give your service technician the engine and generator models, serial numbers, duty rating, applied-load history, photographs, and alarms. Agree on the checks needed to distinguish an operating issue from a mechanical fault.
- Agree on a test plan. Where loaded operation is appropriate, the plan should identify the load steps, duration, monitored temperatures, and stopping criteria for your equipment. Cummins recommends increasing load over time to reduce wet-stacking deposits.3
- Provide a suitable load. Your team can use an approved facility-load test or a supplemental load bank. A load bank provides a test load when you cannot use critical facility equipment.3
- Review the results. Retain the load, temperature, alarm, and inspection records. If symptoms remain, return to the diagnosis rather than repeating a burn-off indefinitely.
- Change the operating plan. Resolve the recurring low-load condition through the approved exercise schedule, supplemental loading, or a revised generator configuration.2,3
A good installation detail: for your load-bank plan to work, you must have access to an electrical connection and working space. When designing the system, allow for temporary load-bank connections; location of connection, cable routing, and responsibility for test equipment should be on drawings that are approved.5
Include the manual revision, service contact, and parts identification as part of your handover package for the ordered configuration; make local service arrangements separately.Make the order manual revision, service contact and parts identification part of your handover package (ordered configuration); make local service arrangements separately.
If you are placing a service contract or purchase order, ask for a test method to be documented and for testing access to be provided, not a blanket assurance that the deposits can be cleared after.
How Can You Prevent Generator Wet Stacking?
Prevent the reoccurrence of wet stacking by ensuring that the engine’s stated low load performance matches your load characteristics and an approved means of ensuring appropriate engine operating parameters exists.4
Check the Exact Model’s Minimum-Load Requirements
The limits should be the manufacturer’s limits for the engine – not a universal percentage from another generator. That’s proof that configuration is important, not that all modern diesel engines are going to run forever unloaded, right?4
Write down minimum load allowed, low load time, temperature readings required, and recovery procedure. If a supplier states that the low-load operation is better, ask him/her to provide the description of the feature and the conditions under which it would be operating the engine and software provided to you.4
Test With a Meaningful Electrical Load
If there is insufficient demand within the facility, loaded exercise or supplemental load-bank testing shall be part of an approved operating plan, records of the testing conducted shall be retained, and the site testing schedule and equipment instructions shall be followed.3 If the demand is not sufficient within the facility, loaded exercise or supplemental load-bank testing shall be a part of an approved operating plan, the records of the testing conducted shall be retained, and the site testing schedule and equipment instructions shall be followed.
To keep a uniform track of the calculation, compute:
The load (%) is the measured electrical output (kW) divided by the applicable rated electrical output (kW) times 100.
This is a ratio calculation, NOT a minimum load recommendation. Write down the duty rating that is used as the denominator, and consider kW vs kVA different: Cummins sees the power of the engine and the kVA of the alternator as different considerations.5
Compare Loading and Generator Configuration Options
The configuration should be selected based on the starting demand as well as the operating conditions. When wet stacking alone does not prove that the generator should be replaced by a smaller one, the motor-starting requirement could be a reason for the additional capacity.5
| Option | Choose it when | Do not choose it when |
|---|---|---|
| Keep the generator and revise loaded testing | The existing generator meets peak requirements and your team can carry out the approved loaded-exercise plan.3 | Your normal duty remains outside the documented operating limits and the revised plan does not address it. |
| Add supplemental loading | Facility load is insufficient or cannot be used for routine testing, and a load bank can be integrated.2,3 | The supplier has not defined how the additional load is controlled and how testing fits your site. |
| Select a smaller generator | A sizing review confirms that a smaller unit can meet running demand, motor starting, and site conditions.5 | Lower rated kW would compromise the required starting or load-step performance.5 |
| Use multiple generators with load-demand control | A properly designed system can take excess capacity offline while retaining the required redundancy.6 | Your site cannot justify the additional controls, integration, and maintenance arrangements. |
We believe our selection judgment is that the low-load generator operating plan should be priced with the generator. A lower cost unit that needs to be serviced regularly and your site can’t support is not the lower cost usable solution for you.
A second judgement relates to future extension: don’t deplete the low load risk today in exchange for an established motor starting need, don’t purchase unused future capacity without a plan to operate. Complete an Industrial Generator Sizing Review to assess the high and low points of your load profile.
Do not include configurations on your shortlist that fall outside the minimum-load conditions or starting performance that is able to be specified for your project.
What Should You Include in a Generator RFQ?
Your RFQ should include a description of the load profile, and a request for the model-specific proof that the generator under consideration can handle the load profile. The following checklist is a suggestion for procurement, not that there is a single universal limit for all fields.
| Specification field | Information you provide | Evidence you require from the supplier |
|---|---|---|
| Duty/electrical supply | Standby or prime duty, voltage, phase, and frequency | Datasheet, including rating and electrical configuration that are offered |
| Sustained load profile | Minimum, typical and maximum running kW; hours at each kW level | Written assessment against the engine’s low-load limits |
| Starting events | Motor starting method, starting demand, and restart sequence | Model-specific starting or load-step assessment |
| Operating environment | Site altitude and ambient-temperature range | Applicable site-rating and operating guidance |
| Low-load procedure | Expected no-load and light-load operation | Minimum load, duration limits, measurement points for temperature, and recovery procedure |
| Engine and controls | Required diagnostics and monitoring | Engine model, controller, software-dependent features, and manual revision |
| If equipped with aftertreatment | Market of use and operating duty | Ordered emission configuration, applicable approval documentation and regeneration instructions |
| Testing access | Connection location, available space, and facility-load restrictions | Connection drawing, protection arrangement, and commissioning test plan |
| Handover and support | Service location and maintenance responsibilities | Test records, manuals, parts list, written warranty terms, and regional support scope |
Link the supplied diesel product line and support material to this particular evidence package for an order with PEGASO.7,8 Make any low load claim conditional until the supplier claims it for the offered engine and configuration.
Your specification should include any supplier’s stated limits and responsibilities, to allow your team to make comparisons.
What is a generator wet stacking?What is meant by generator wet stacking?
These answers can help you filter out overall wet-stacking advice for what information you need for your generator.
Frequently Asked Questions About Generator Wet Stacking
You can use these answers to separate general wet-stacking guidance from the evidence needed for your generator.
How can I prevent wet stacking in my generator?
Wet stacking is a problem mainly associated with diesel generators that run for long periods at very light loads. The engine does not get hot enough to fully burn the fuel, so unburned fuel, soot, and oil residue can collect in the exhaust system. It may show up as a thick black oily residue at exhaust joints or heavy soot buildup.
To prevent wet stacking:
- Avoid prolonged low-load operation. Try not to run a diesel generator for long periods below about 30% of its rated load unless the manufacturer specifically allows it.
- Exercise the generator under load. A routine loaded run helps raise cylinder and exhaust temperatures so deposits burn off. Many maintenance programs use a load of roughly 30–75%+ of rated capacity, depending on the engine manufacturer’s guidance and application.
- Use a load bank when the normal electrical demand is too small. A load bank adds an artificial electrical load so the engine operates at a healthier temperature.
- Size the generator correctly. An oversized generator that spends most of its life lightly loaded is much more likely to develop wet stacking.
- Do not let the generator idle unnecessarily. Extended idling or “warming up” with little or no load can contribute to carbon and fuel buildup.
- Maintain the engine properly. Keep air filters, fuel systems, injectors, and cooling systems in good condition. Poor combustion from restricted air or fuel-system issues can worsen deposits.
- Monitor exhaust signs. Look for black oily leakage around exhaust connections, unusual smoke, or declining performance. These can indicate wet stacking or related combustion issues.
If you suspect your generator already has wet stacking, a common corrective action is a properly controlled high-load run (often with a load bank) to bring the engine to operating temperature and burn off deposits. If deposits have caused cylinder glazing or other damage, a load run alone may not fix it.
What are the signs that my generator is experiencing wet stacking?
Wet stacking occurs when a diesel generator runs under light loads (typically less than 30% of its capacity) or idles for too long, preventing the engine from reaching its optimal operating temperature. Because the combustion chamber isn’t hot enough, fuel doesn’t burn completely, and the unburned residue mixes with soot and carbon, routing into the exhaust system.
The primary warning signs of wet stacking include:
- Black, oily ooze: A thick, tar-like, or black liquid leaking or weeping from exhaust manifold joints, turbocharger housings, or exhaust pipe connections. This fluid is often mistaken for engine oil, but it is actually unburned fuel mixed with carbon soot.
- Heavy black smoke: Puffing or blowing dark gray/black smoke from the exhaust stack, particularly right when the generator is first put under a heavy load and burns off the accumulated backlog of fuel.
- Reduced power output: A noticeable drop in the generator’s performance and efficiency, meaning it struggles to meet its rated capacity or power demands.
- Rough or uneven running: An audible engine miss or sputtering during operation, which happens when fuel injectors become fouled or caked with carbon buildup.
- Increased fuel and oil consumption: The unit burns more fuel to achieve the same output, and unburned fuel can blow past piston rings to dilute the crankcase oil—leading to higher overall oil consumption.
- Lower exhaust temperature: A measurable or noticeable drop in overall exhaust temperatures during regular operation because combustion is incomplete.
What causes wet stacking in diesel generators?
Wet stacking in diesel generators is an accumulation of unburned fuel, carbon soot, condensed water, and oil residues in the exhaust system . It happens when a diesel engine fails to reach or maintain its optimal operating temperature, leading to incomplete combustion.
According to explanations from Wikipedia on Wet Stacking and industry insights via W Power Products , the primary underlying causes include:
- Running under light loads: Operating a generator at a low percentage of its capacity—typically under 30% to 60% of its rated output—prevents the combustion chamber from getting hot enough to burn fuel completely.
- Prolonged idling or light cycling: Running the engine at rated speed with little to no electrical load applied fails to generate the thermal energy required in the cylinders.
- Oversized generators: Using a generator unit that is vastly larger than the actual electrical demand means the engine rarely sees a heavy enough load to operate efficiently.
- Faulty fuel injectors: Leaking, worn, or clogged injectors fail to atomize the diesel fuel into a fine mist, dumping raw fuel droplets into the cylinder instead.
- Low combustion chamber pressure: Inadequate heat or worn piston rings prevent a proper seal, allowing unburned fuel and oil to slip past into the exhaust.
- Cold weather operations: Running the engine in freezing ambient temperatures without proper warm-up or coolant thermostats keeps the engine block below optimal operating thresholds.
- Restricted airflow: A clogged air filter or a failing turbocharger disrupts the proper air-to-fuel ratio, starving the combustion process of necessary oxygen.
Are there effective ways to clean or fix wet stacking in diesel engines?
The most effective way to clean and fix wet stacking in a diesel engine is to perform a load bank test or apply a heavy electrical load (typically 70% to 80% capacity) for 2 to 4 hours to raise exhaust temperatures and burn off accumulated unburned fuel and carbon deposits.
Sources
- Caterpillar, Wet Stacking? What Is That?, Power Bytes transcript. Supports exhaust-deposit composition, visible symptoms, and potential performance and wear consequences. Accessed September 30, 2026.
- Caterpillar, The Impact of Generator Set Underloading, publication LEXE0832-01. Supports prolonged underloading, cold operating conditions, aftertreatment concerns, and load-management planning. The document’s guidance is not a substitute for the offered model’s manual. Accessed September 30, 2026.
- Cummins Sales and Service, Load Bank Performance Testing. Supports loaded exercise, supplemental load banks, and increasing load over time to reduce wet-stacking effects. Accessed September 30, 2026.
- Caterpillar, Wet Stacking Prevention Technologies for Cat 3500 Diesel Generator Sets. Supports engine-specific cylinder-cutout strategies and their enablement conditions; these features are not attributed to PEGASO products. Accessed September 30, 2026.
- Cummins Power Generation, Application Manual: Liquid Cooled Generator Sets, A040S369, Issue 9, February 2015; sections 2.6, 3.3.3, and 5.2. Supports installation access, load-bank connection planning, motor-starting considerations, and kW/kVA sizing. Accessed September 30, 2026.
- Cummins, Diesel Generators in the Data Center: Frequently Asked Questions, bulletin 5544090, June 2018. Supports load-demand control and preserving defined redundancy in a parallel system. Accessed September 30, 2026.
- PEGASO, Silent Diesel Generator Collection. Supplier-published evidence of a diesel product family, not independent proof of any model’s minimum-load capability or certification. Accessed September 30, 2026.
- PEGASO, Support Resources. Supplier-published support and documentation scope; model-specific documents and regional commitments require confirmation. Accessed September 30, 2026.
