Symptom guide
On this page
- What it means
- When black smoke happens matters
- At cold start only
- Throughout warm operation
- Only under hard acceleration
- On diesel engines
- Common causes ranked (gas engines)
- How to diagnose it, in order
- 1. Pull codes
- 2. Visual inspection
- 3. Live fuel trim data
- 4. Check ECT
- 5. MAF inspection and cleaning
- 6. Fuel pressure test
- 7. Injector test
- Fixes, cheapest first
- How to reset
- What to do if it continues
- Related guides
Black Smoke From Exhaust
What it means
Black smoke is unburned carbon — the engine is putting more fuel into the cylinders than it can burn cleanly. The excess fuel carbonizes in the exhaust and exits as black soot.
Different colors mean different things:
| Smoke color | Cause |
|---|---|
| Black | Rich (too much fuel) |
| Blue/gray | Burning oil |
| White (warm) | Burning coolant |
| White (cold start, 30 sec) | Normal condensation |
When black smoke happens matters
At cold start only
Normal in cold weather for the first 30–60 seconds. The PCM intentionally runs slightly rich during cold-start enrichment for faster catalyst light-off. Black smoke clearing within 60 seconds is normal.
Throughout warm operation
Confirmed rich condition. Diagnose.
Only under hard acceleration
Power enrichment is normal during wide-open throttle (PCM commands slightly rich for maximum power). Brief black smoke at WOT is normal. Persistent black smoke at WOT means more than just enrichment — actual leak or fault.
On diesel engines
Diesels naturally emit more visible exhaust than gas engines under load. Black smoke from a diesel can be:
- Normal under hard acceleration: smoke during full-load acceleration is somewhat normal.
- Excessive smoke during normal driving: turbo issue, EGR issue, or fuel system problem.
Common causes ranked (gas engines)
1. Clogged air filter (~25%). Air can't enter properly; PCM keeps adding the expected fuel amount. Mixture turns rich. Clue: visible heavy contamination; filter past 30,000 miles unchanged.
2. Contaminated or oil-fouled MAF sensor (~10%). MAF over- reports airflow; PCM adds matching fuel. Mixture rich. Clue: MAF contaminated by oily aftermarket K&N filter.
3. Leaking fuel injector (~25%). Injector drips fuel between
firings. Cylinder gets too much fuel. Clue: one plug significantly
darker than others; P0172 and possibly
P0301–P0306.
4. Fuel pressure regulator stuck high (~15%). Rail pressure above spec; all injectors deliver more fuel per pulse. Clue: mechanical gauge confirms high pressure.
5. ECT sensor stuck cold (~10%). PCM thinks engine is
permanently cold, runs cold-start enrichment continuously. Clue:
P0118 stored; live data shows abnormally low
ECT.
6. Turbo boost leak (turbo engines) (~5%). Boost-pipe leak
post-MAF causes lean condition but PCM compensates by adding fuel
incorrectly. Clue: P0299 or P0171.
7. Failed evap purge valve (stuck open) (~5%). Constant vapor
flow into intake at all times. Clue: P0496.
8. Failed PCV system (~3%). Excessive crankcase vapor adding unmetered fuel-air mixture. Clue: visible oil at PCV hose.
9. Failed O2 sensor (~2%). Sensor reports false lean, PCM keeps
adding fuel. Clue: P0131 or P0132.
How to diagnose it, in order
1. Pull codes
P0172 (rich Bank 1), P0175
(rich Bank 2), or no codes (early stage rich without trip).
2. Visual inspection
- Air filter: pull and inspect. Dust-loaded = replace.
- Spark plugs: all soot-black = global rich. One darker = that cylinder's injector.
3. Live fuel trim data
LTFT (long-term fuel trim) on scan tool:
- Negative LTFT (-15% or more): PCM trying to reduce fuel. Real rich condition.
- Below zero but slight (-5% to -10%): mild rich; could be filter or MAF.
4. Check ECT
If LTFT very negative AND ECT live data shows cold reading after warm-up = ECT sensor stuck cold. Replace.
5. MAF inspection and cleaning
If LTFT slightly negative and the air filter is clean, MAF cleaning is the next test. CRC 05110 only.
6. Fuel pressure test
Mechanical gauge at the rail Schrader port. High pressure (>10 psi above spec) = stuck regulator.
7. Injector test
Cylinder balance test on scan tool. A cylinder that fails balance points at the leaking injector.
Fixes, cheapest first
| Fix | Cost (USD) | When it applies |
|---|---|---|
| Replace air filter | $10–$40 | Filter visibly contaminated |
| Clean MAF sensor | $10 | MAF oil-fouled or contaminated |
| Replace MAF sensor | $80–$300 | Cleaning doesn't help |
| Replace ECT sensor | $20–$60 | ECT live data abnormal |
| Replace one fuel injector | $40–$200 part + 30 min | Specific cylinder failed |
| Replace all injectors | $300–$800 | High-mileage or contaminated fuel |
| Replace fuel pressure regulator | $50–$200 | Pressure high |
| Replace purge valve | $20–$120 | Stuck open |
| Replace catalytic converter (if damaged) | $400–$2,500 | Months of rich operation |
How to reset
Clear codes after the fix, drive 100 miles of mixed conditions. Fuel trims relearn during normal operation. LTFT should normalize toward zero within 50–100 miles.
What to do if it continues
- After air filter + MAF cleaning: focus on fuel system (regulator and injectors).
- After fuel system work: check ECT and PCV.
- After everything: O2 sensor or PCM.