Short answer
Diesels like work. When one runs for long periods at very light load, like an oversized generator or an engine left idling, combustion temperatures stay too low. Unburned fuel and carbon collect in the exhaust, often seen as black, oily residue at the stack. Generator makers call this wet stacking.
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Start with the easy checks
- Look for black oily residue or drips at the exhaust outlet.
- Compare typical load to the generator rating; follow the maker's minimum load guidance.
- Cut unnecessary idling on trucks.
Likely causes, most common first
- Running underloaded
- The main cause.
- Long idling
- Same effect on vehicles.
- Oversized generator for the job
- Never gets enough load.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
Generator makers recommend running units with enough load during exercise, and using a load bank (a device that applies artificial electrical load) when building load is not practical.
When to stop and see a diesel tech
- Heavy residue, smoke or power loss.
- Standby generators that only ever run unloaded: have a generator tech review the exercise plan.
Questions
How do you prevent wet stacking?
Run the generator with adequate load per the manufacturer, avoid long unloaded runs, and use a load bank for testing if needed.
Related symptoms
What does white smoke from my diesel mean?
Troubleshoot →Fuel systemWhy is my diesel oil level rising, and does it smell like fuel?
Troubleshoot →EmissionsWhat is DPF regeneration and why does my truck keep doing it?
Troubleshoot →Where this comes from
- Caterpillar: Wet stacking, what is that?
- Generac: Wet stacking white paper (PDF)
- Avtron Power: Adverse effects of low-load operation on generator sets (PDF)
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.