Quick answer
Record the generator model, rating basis, actual kW and kVA, percent load, duration, ambient conditions, coolant or jacket-water condition where applicable, exercise schedule, exhaust observation, oil level, alarms, and recent maintenance. Caterpillar identifies extended operation below roughly 30 percent load as a wet-stacking concern for the equipment discussed in its guidance, but the controlling minimum-load and remediation instructions are model-specific. Use the manufacturer and qualified service provider to correct the load profile and plan any load-bank test.
Recognize wet stacking without assuming every dark leak is harmless
Caterpillar describes wet stacking as thick black liquid that can appear at exhaust manifold, turbocharger, or pipe joints after extended low-load operation. The material is associated with carbon and unburned fuel and can resemble used lubricating oil.
Exhaust leakage, turbocharger problems, injector faults, oil consumption, coolant loss, and other engine conditions can produce overlapping observations. Stop for fire risk, abnormal smoke, rising oil level, loss of power, high temperature, low oil pressure, fuel leakage, or any manufacturer alarm. Qualified inspection must identify the actual source.
Calculate the real load percentage over time
Use measured real power and the applicable prime, standby, or continuous rating for the operating duty. Record load at useful intervals instead of relying on breaker position or connected nameplate totals. Include unloaded exercise, overnight minimums, seasonal periods, redundant sets, and load-shed operation.
A generator sized for the maximum event can spend most hours at a much lower load. Oversizing, staged facility growth, redundant capacity, and lightly loaded exercise can all contribute. The solution may involve controls, sequencing, right-sizing, paralleling, supplemental loading, or operating changes, but it must preserve maximum-demand and reliability requirements.
| Record | Why it matters | Source |
|---|---|---|
| Measured kW and kVA | Defines actual loading and power factor | Calibrated system data |
| Rating and duty | Sets the correct percentage basis | Exact generator data sheet |
| Duration and temperature | Shows whether low load is sustained | Controller and site log |
| Exhaust and oil observations | Preserves symptom evidence | Inspection and service report |
Prevent chronic underloading through system operation
Review automatic start and stop logic, exercise behavior, number of sets online, load-management priorities, facility growth assumptions, and minimum nighttime or seasonal demand. A successful start does not prove that an unloaded exercise is the right long-term readiness program.
Do not add improvised heaters or uncontrolled load solely to raise percentage. Supplemental or load-bank equipment needs suitable ratings, protection, ventilation, controls, connection hardware, fuel capacity, and a written failure response.
Use a qualified remediation and verification plan
The engine manufacturer may prescribe inspection, oil and filter checks, injector or turbocharger assessment, exhaust cleanup, loaded operation, or a controlled load-bank test. A generic instruction to run at full load can exceed a site, alternator, cooling, fuel, or facility limit.
After correction, trend load, exhaust condition, oil behavior, alarms, and maintenance. Update the sizing model before replacing a generator with another set of the same oversized rating. Preserve the test report and define when the next review is triggered.
Frequently asked questions
Common questions
What causes diesel generator wet stacking?
A common cause is extended operation at low load, which can keep combustion and exhaust temperatures too low. Engine faults and site conditions still require inspection.
What does wet stacking look like?
Caterpillar describes thick black liquid at exhaust-system joints, sometimes called exhaust slobber. Do not assume every dark leak has the same cause.
Is 30 percent load a universal minimum?
No. It is a common threshold in Caterpillar guidance for the discussed equipment, but the exact model, duty, engine, temperature, and manufacturer instructions control.
Can a load bank fix wet stacking?
A qualified load test may be part of the manufacturer-approved response, but it does not correct an underlying oversized or chronically underloaded operating plan by itself.
Primary references
CordFind summarizes connector specifications and public safety guidance. Always follow the instructions supplied with your equipment.