Wet stacking is a condition where unburned diesel fuel accumulates inside a generator’s exhaust system, typically because the engine has been running at too light a load for too long. The name comes from the visible result: a thick, dark liquid that drips or oozes from the exhaust pipes (the “stacks”), making them look wet. It’s one of the most common problems with standby diesel generators, and it’s almost entirely preventable.
How Wet Stacking Happens
Diesel engines are designed to run under a substantial load. When a generator operates below about 30% of its rated output for extended periods, the engine never reaches its optimal operating temperature. At lower temperatures, diesel fuel doesn’t combust completely. The unburned fuel vapors pass into the exhaust system, where they condense and mix with soot to form a dark, oily substance that coats internal components.
This residue collects on the combustion chamber walls, injector nozzles, piston rings, the turbocharger, and throughout the exhaust system. Over time, the buildup compounds. Carbon deposits foul the injectors, reducing combustion efficiency further, which creates a cycle where the engine runs even less cleanly. The turbocharger can lose efficiency as residue coats its internals, and piston rings can lose their seal against the cylinder walls, a process sometimes called cylinder glazing.
Standby generators are especially vulnerable because they often sit idle for weeks or months, then run only during brief tests with little or no real electrical load connected. A generator humming along at 10% or 15% of capacity during a monthly test is a prime candidate for wet stacking.
How to Spot It
The most obvious sign is a thick, dark liquid dripping from the exhaust outlets. It looks similar to engine oil but is actually condensed fuel vapor mixed with soot. You may also see it oozing from the turbocharger housing or collecting around exhaust joints. Black smoke from the exhaust during startup or light-load operation is another indicator, as are soot-stained exhaust pipes.
Less visible symptoms include reduced power output, rough running, and increased fuel consumption. If your generator struggles to pick up load or sounds rougher than it used to during testing, accumulated carbon deposits from chronic light-load operation could be the cause.
What Load Levels Are Safe
The threshold varies depending on fuel type. For diesel generators, operating below 30% of rated output for extended periods is where problems start. Standby and prime-rated diesel units are designed to run between 50% and 85% of their nameplate rating. Running consistently within that range keeps exhaust temperatures high enough to burn fuel completely.
Natural gas generators are even more sensitive. Caterpillar, one of the largest generator manufacturers, recommends never loading a natural gas unit below 50% of rated output for any duration. The ideal operating range for natural gas units is 70% and above. At reduced loads, the time limits are strict: a natural gas generator running at 0% to 30% load should not continue beyond 30 minutes, and at 31% to 50% load, the limit is two hours. After that reduced-load period, the engine needs at least two hours at 70% load or higher to recover.
For diesel units, Caterpillar recommends loading the generator to at least 30% for approximately 30 minutes for every four hours of light-load operation. That minimum is enough to raise exhaust temperatures and burn off accumulated fuel and carbon.
How Load Bank Testing Fixes It
A load bank is a device that creates an artificial electrical load for a generator to work against. It converts the generator’s electrical output into heat, allowing you to run the engine at any load level you choose, regardless of whether there’s actual demand from a building or facility. This is the standard tool for both preventing and reversing wet stacking.
When you connect a load bank and bring the generator up to full rated output, exhaust temperatures climb high enough to burn off the unburned fuel and carbon deposits that have accumulated. Think of it like taking a car that’s been idling in a parking lot out on the highway for a sustained drive. The higher temperatures clean out the buildup.
For generators that have been wet stacking for a while, a load bank session at full rated capacity for one to four hours can often restore normal operation. In severe cases where deposits have hardened or cylinder glazing has occurred, mechanical cleaning or component replacement may be necessary before a load bank session can fully resolve the issue.
Testing Schedules That Prevent Problems
NFPA 110, the standard that governs emergency and standby power systems for hospitals, data centers, and similar facilities, provides a useful framework even if your generator isn’t subject to that code. The standard recommends monthly operational testing at a minimum of 30% of the generator’s nameplate rating, or running long enough for the engine to reach normal operating temperatures.
Beyond the monthly tests, NFPA 110 calls for an annual load bank test at the generator’s full rated load, typically lasting 1.5 to 4 hours. Facilities with critical operations, like hospitals and data centers, often perform load bank testing quarterly or semi-annually.
If your generator serves as backup power for a commercial building, following a similar schedule is a practical way to keep wet stacking from ever becoming an issue. Monthly tests under meaningful load, combined with an annual full-load session using a load bank, will keep exhaust temperatures where they need to be and prevent the slow accumulation of unburned fuel.
Sizing Generators Correctly
Oversized generators are the root cause of many wet stacking problems. If a facility installs a 500 kW generator but its actual emergency load is only 100 kW, that generator will spend nearly all its running time at 20% capacity, well below the safe threshold. This is common when generators are sized for future expansion that never materializes, or when a building’s electrical load drops after equipment upgrades or tenant changes.
If you’re purchasing a new generator, matching the unit’s rating closely to your actual load requirements is the simplest long-term prevention. For existing generators that are significantly oversized for their load, regular load bank testing becomes essential rather than optional. Some facilities add a permanently installed load bank that automatically engages when the generator’s real load drops below a set threshold, keeping the engine in a safe operating range without any manual intervention.