Black smoke from a diesel means fuel is not burning completely, almost always because there is too much fuel for the available air. The common causes are a restricted air filter, a boost leak, a failing turbo, dirty or leaking injectors, and an EGR system that is stuck or clogged. A short puff of black smoke when accelerating hard is common on many older diesels; heavy, continuous black smoke is not.
Causes of diesel black smoke
| Cause | Clues |
|---|---|
| Clogged air filter or intake | Smoke under load, gradual power loss |
| Boost leak | Hissing under boost, underboost code P0299 |
| Turbo problems | Low boost, oil in the intake, whistle or grinding; see turbo failure symptoms |
| Injectors | Rough idle, smoke at idle and under load; see bad diesel injector symptoms |
| Stuck EGR valve | Smoke and sluggishness, especially at part throttle |
| Low compression | Smoke with blow-by; see diesel blow-by |
Black smoke under load or on acceleration
Under load, the engine needs the most air. A small boost leak at a hose clamp or intercooler boot, a dirty air filter or a turbo that is not making full boost shows up here first. Pressure-test the intake and charge air system.
Black smoke on startup or when cold
A brief cloud on a cold start can be normal on older engines. Lasting smoke after a cold start points to injectors or low compression. White or gray smoke when cold is a different problem; see diesel white smoke and glow plug symptoms.
Black smoke at idle
Smoke at idle usually means an injector is dripping or stuck, since airflow at idle is plenty for the small amount of fuel needed.
How to fix it
- Replace the air filter and inspect the intake for blockages.
- Pressure-test the boost system for leaks.
- Check turbo boost and look for shaft play.
- Read codes and injector balance rates.
- Inspect and clean or replace a sticking EGR valve.
Black smoke after other repairs
If black smoke started after work on the intake, intercooler or turbo, look first for a boot or clamp that was not fully seated, a cracked charge pipe, or a sensor left unplugged. A disconnected boost or mass airflow sensor can make the engine computer miscalculate fuel. Smoke after an injector replacement can mean injector codes were not entered or a washer was missing.
Why it matters
Black smoke wastes fuel, raises exhaust temperatures and loads the emissions system with soot. On newer trucks, excessive soot means more frequent particulate filter regenerations. See also exhaust smoke colors, diesel losing power and our Cummins engines.
Trouble codes: P2263 Boost System Performance · all OBD2 codes.
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