White Smoke or Rough Running on Cold Start — LLY / LBZ / LMM Duramax
White smoke or rough running that clears as the engine warms on LLY/LBZ/LMM Duramax 2004.5–2010 usually means cold-start combustion is incomplete. The platform…
What’s going on
On LLY/LBZ/LMM Duramax, white smoke and rough running on cold start typically indicate incomplete combustion during cold start, most often from weak or missing glow-plug heat or an engine that never reaches normal temperature. The symptom clears as the engine warms.12
Most likely causes
- Glow plug or glow plug control module fault — Weak or inoperative glow plugs or a faulty glow plug control module reduce cold-start combustion heat, causing white smoke and rough idle that improves with engine temperature. Related trouble codes: P0380, P0670.13
- Engine running too cold due to thermostat stuck open — A mechanical thermostat stuck open prevents the engine from reaching operating temperature in the expected time, leading to prolonged white smoke and rough running on cold start. Related trouble code: P0128.12
How to narrow it down
- Read stored trouble codes first; P0380, P0670 point to glow-plug system faults, P0128 points to thermostat performance.
- Check glow plug system operation and control module output during cold start; verify glow plug resistance and supply voltage.
- Monitor engine coolant temperature rise from cold start with a scan tool; compare to expected warm-up time and normal operating temperature.
- Inspect thermostat function and coolant temperature data; confirm the engine reaches and maintains normal operating temperature.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| Battery and charging voltage | 12.6–12.7 V at rest when fully charged; about 13.5 V running | Engine off, then running with loads on | DieselHub |
| Glow plug resistance | Low — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plug | Engine off, meter lead to engine ground | DENSO |
Common mistake to avoid
Replacing glow plugs or the glow module before checking the batteries. The glow system draws heavily, and low battery voltage causes circuit faults that no part swap will fix. Source: FleetRabbit — P0670 glow plug control module fault.
Parts and tools you may need
- Scan tool with live data
- Digital multimeter
- Glow plug test light
- Infrared thermometer
Tools you’ll need
- OBD-II scan tool — Read P0380/P0670/P0128 codes and monitor live coolant temperature rise during cold start.
- Digital multimeter — Measure glow plug resistance and control module supply voltage to confirm weak or missing heat.
- Infrared thermometer — Verify actual engine/coolant temperature rise and thermostat warm-up behavior.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Thermostat | $50–$150 | 1–3 hrs | DIY with tools and a manual |
| Glow plugs (full set) | $80–$250 | 2–6 hrs | DIY with tools and a manual |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If white smoke persists after warm-up, or if fuel in oil, excessive crankcase pressure, or injector balance issues are suspected, have the engine evaluated by a qualified diesel technician. Do not attempt high-pressure fuel system diagnosis without proper training and equipment.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Related trouble codes
- P0380 — Glow Plug/Heater Circuit Malfunction
- P0670 — Glow Plug Control Module Circuit
- P0128 — Coolant Thermostat Below Regulating Temperature
Sources
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