Field guide
White, Black or Blue: What the Color of Diesel Exhaust Smoke Tells You
Published · 6 min read
Smoke is a diesel's way of telling you that something other than a clean burn of fuel and air is going out of the tailpipe. The color tells you what that something is. White is mostly liquid that did not burn, black is fuel that burned badly, and blue is oil.
This guide puts the three side by side, with the failures our records tie to each color on each engine, and what to check before anything else.
- White smoke62
- Black smoke30
- Blue or gray smoke11
Power Stroke 7.3 to 6.7, Cummins 5.9 and 6.7, Duramax LB7 to L5P, pulled 2026-09-07 to 2026-10-16. Keyword matches; most owners describe a failure, not the smoke, so these counts are small.
Coolant gets into the cylinders through a failed EGR cooler, a head gasket or a cracked head, and leaves as thick white smoke that often smells sweet. The coolant level in the reservoir keeps dropping with no leak on the ground. aggregated
Unburned fuel looks similar but smells of raw diesel and stings the eyes. It shows on cold starts, when glow plugs, the grid heater or an injector are not doing their job, and it fades as the engine warms. aggregated
A thin vapor on a cold morning that is gone in a few minutes is usually condensation in the exhaust, not a fault.
Check first: the coolant level on a cold engine, then again after a drive; the codes; whether it smells sweet or of diesel.
"[EGR] cooler failure, white smoke coming out of the exhaust pipe caused by coolant entering into the emission system."
Owner of a 2007 F-250 6.0, 2014. NHTSA complaint 10604676
| Engine | Documented failure | What the smoke comes from |
|---|---|---|
| 6.7 Power Stroke | Intercooler condensation ingestion + cold-side charge-pipe fatigue | Boost or turbo problem |
| 3.0 Duramax | EGR system failures: valve, cooler, wiring (derate-prone) | Coolant getting into the cylinders |
| 3.2 Power Stroke (Transit) | EGR cooler failure: overheating and coolant loss (three Ford field actions) | Coolant getting into the cylinders |
| 6.0 Power Stroke | EGR cooler failure | Coolant getting into the cylinders |
| 6.0 Power Stroke | Head gasket failure / cylinder head lift | Coolant getting into the cylinders |
| 6.4 Power Stroke | EGR cooler failure -> #8 hydrolock | Coolant getting into the cylinders |
| 6.7 Cummins 2007.5-2018 | EGR cooler failure | Coolant getting into the cylinders |
| 6.7 Cummins 2007.5-2018 | Head gasket failure (tuned / overheated trucks) | Coolant getting into the cylinders |
| 6.7 Cummins 2019+ | EGR cooler failure (2019+ 6.7) | Coolant getting into the cylinders |
| Duramax LB7-LMM | EGR cooler clogging / failure (LB7-LMM) | Coolant getting into the cylinders |
| Ram 1500 EcoDiesel | EGR cooler cracking and coolant loss (recall 19V-757) | Coolant getting into the cylinders |
| Sprinter 2.1 | EGR system: coking and cooler coolant leak | Coolant getting into the cylinders |
| 6.7 Cummins 2007.5-2018 | DPF clogging / regeneration problems | DPF and regeneration |
| 5.9 Cummins | Common-rail injector wear / nozzle carbon (03-07) | Fuel not burning completely |
| 5.9 Cummins | Intake grid heater / relay failures (cold starts) | Fuel not burning completely |
| 6.0 Power Stroke | Fuel injector stiction / failure | Fuel not burning completely |
| 6.7 Cummins 2019+ | Grid heater bolt and relay failures (2019+ 6.7) | Fuel not burning completely |
| 6.7 Cummins 2019+ | Common-rail injector wear (2019+ 6.7) | Fuel not burning completely |
| 6.7 Power Stroke | Piezo injector wear and fuel dilution of engine oil | Fuel not burning completely |
| 6.7 Power Stroke | Glow plug wear and cold-start faults, plus an isolated 2011-2013 tip-separation defect | Fuel not burning completely |
| 7.3 Power Stroke | Glow plug / glow plug relay failure | Fuel not burning completely |
| Duramax LB7-LMM | LB7 injector failures (the defining repair of the generation) | Fuel not burning completely |
| Sprinter 3.0 V6 | Glow plug module and cold-start roughness | Fuel not burning completely |
23 failure records on 13 engines list this color among their signs; the same list as our white smoke page.
Too little air: a turbo that is not building boost, often a sticking variable-geometry turbo or its actuator, or a leaking charge pipe. Black smoke under load with poor power is the classic sign. aggregated
Too much or badly sprayed fuel: a worn or leaking injector that dribbles instead of spraying. aggregated
On a truck with a DPF, the filter should catch soot before it reaches the tailpipe. Visible black smoke from a DPF truck is worth having the filter checked.
Check first: codes for boost (P0299 on many engines) and injectors; the air filter; the charge pipes and their clamps.
"Sometimes when you accelerate, there is little to no response except black smoke billowing from the exhaust."
Owner of a 2008 Ram 2500 6.7 Cummins, 2009. NHTSA complaint 10286925
| Engine | Documented failure | What the smoke comes from |
|---|---|---|
| 6.7 Cummins 2007.5-2018 | Holset VGT turbo sticking / actuator failure | Boost or turbo problem |
| Duramax LB7-LMM | VGT turbo vane carbon / actuator wear (LLY+) | Boost or turbo problem |
| 5.9 Cummins | 12v overflow valve weak (hard start / low power) | Fuel not burning completely |
| 6.7 Power Stroke | Piezo injector wear and fuel dilution of engine oil | Fuel not burning completely |
| Duramax L5P | Injector wear / pigtail-connector misfires | Fuel not burning completely |
5 failure records on 5 engines list this color among their signs; the same list as our black smoke page.
Oil reaches the cylinders past worn piston rings or valve guides, or the exhaust past failed turbo seals. On older injectors sealed with o-rings, a failed o-ring can do the same. Blue smoke usually comes with oil use you can see on the dipstick. aggregated
Check first: how much oil the engine uses between changes; oil in the intake or charge pipes, which points at the turbo; a compression or leak-down test if the oil use is high.
"When accelerate from light/sign, grey/blue smoke streams from vehicle."
Owner of a 2010 F-450 6.4, 2010. NHTSA complaint 10357696
| Engine | Documented failure | What the smoke comes from |
|---|---|---|
| 7.3 Power Stroke | Injector o-ring failure | Fuel not burning completely |
| 6.4 Power Stroke | Piston ring failure #7/#8 (regen fuel wash) | Oil getting into the cylinders |
| 6.4 Power Stroke | Valve guide wear (no bronze sleeves) | Oil getting into the cylinders |
| 6.7 Power Stroke | VGT turbocharger failure - generation dependent (ceramic-bearing GT32 vs journal-bearing GT37+) | Oil getting past the turbo seals |
4 failure records on 3 engines list this color among their signs; the same list as our blue smoke page.
Smoke that does not mean a fault
- A short white puff at a cold start that clears quickly, especially on a damp morning.
- A brief puff of black under hard acceleration on an older diesel without a DPF.
- Steam from the tailpipe in winter after a short drive, from water in the exhaust.
Anything thick, lasting or new deserves a look, and white smoke with a falling coolant level deserves one before the next drive. On a 6.0 or 6.4 Power Stroke, our myth check on the 6.0 explains why coolant loss and white smoke are the start of the most expensive chain on those engines.
What we cannot tell you
- Your exact cause from the color alone. The color narrows it to a kind of problem; the codes, the fluid levels and a shop test find the part.
- How common each color is. Most owners describe the failure, not the smoke, so only a hundred or so complaints name a color at all.
FAQ
What does white smoke from a diesel mean?
Usually liquid that did not burn: coolant from a failed EGR cooler or head gasket (sweet smell, falling coolant level), or raw fuel from a cold-start or injector problem (diesel smell, fades as the engine warms). A thin vapor on a cold morning that clears in minutes is normal.
What causes black smoke from a diesel truck?
Too much fuel for the air: low boost from a turbo or charge pipe problem, or a leaking injector. On a truck with a DPF, the filter should catch the soot, so black smoke at the tailpipe is also a reason to have the filter checked.
Is blue smoke from a diesel bad?
It means the engine is burning oil, past the rings, the valve guides or the turbo seals. Check how much oil it uses, and have the turbo and compression checked if it is more than a little.
Go deeper
- Symptoms, engine by engine: White smoke · Black smoke · Blue or gray smoke · Coolant loss
- How a modern diesel works, in one diagram · P0299 and low boost on the 6.7 Power Stroke
- Buyer's checklists by engine
Sources. Our failure records for each engine, as listed on the symptom pages linked above. NHTSA owner complaints (US government public domain) on ten diesel engine families, keyword-matched; quotes normalized for capitalization, with one word in brackets where the owner misspelled it. Torque Index takes no paid placement.