Rough Idle and White Smoke on Cold Start — 6.7 Cummins (EGR + DPF, pre-SCR)

Guide for 6.7 Cummins (EGR + DPF, pre-SCR) 2007.5–2012 showing rough idle and white smoke on cold start.

Updated October 15, 2026

What’s going on

On this engine, rough idle with white smoke on a cold start usually means the intake air is not being heated enough for clean combustion, an injector is not firing cleanly when cold, or the engine is running too cold because the cooling system is not reaching normal temperature. The 6.7L uses an intake-air grid heater, not glow plugs. White smoke and rough running when cold usually means the grid heater or its relays are not working, an injector is not firing cleanly, or the engine is running too cold.¹

Most likely causes

1. Intake air grid heater system not heating — The grid heater is commanded on cold starts to raise intake air temperature. If the heater element, relays, heavy cables or connections are degraded, intake air stays cold, causing rough idle and white smoke until the engine warms. This condition commonly sets P2609.¹²
2. Injector not delivering clean fuel when cold — A common-rail solenoid injector that dribbles or has poor spray pattern will cause a cold misfire and white smoke. On this platform injector electrical or performance faults are in the P0263 family.¹
3. Engine running too cold due to cooling system — A stuck-open mechanical thermostat prevents the engine from reaching normal operating temperature, prolonging cold-start roughness and white smoke. This typically sets P0128.¹

How to narrow it down

1. Read stored trouble codes first. P2609 points to grid heater system performance; P0263-family points to injector performance; P0128 points to coolant temperature below thermostat regulating temperature.
2. Check intake air grid heater operation on a cold soak. Verify battery voltage at the heater relays and the heater element, and confirm the heater draws current when commanded. Inspect heavy cables and connections for corrosion or heat damage.
3. Monitor engine coolant temperature from cold start with a scan tool. Compare rise rate and stabilized temperature to spec for a mechanical thermostat. A slow rise or temperature that never reaches normal suggests a stuck-open thermostat.
4. Perform a cold-start injector balance test using scan data. Look for a cylinder with low contribution, high correction, or rough running that improves as the engine warms, indicating a cold injector fault.

Specs and test values

Parts and tools you may need

• Scan tool with live data and bi-directional control
• Digital multimeter and clamp ammeter
• Battery load tester
• Infrared thermometer
• Basic hand tools for relay and grid heater access

Tools you’ll need

• OBD-II scan tool — Reads P2609/P0263/P0128 and provides live coolant temperature and injector balance data for cold-start diagnosis.
• Digital multimeter — Verifies battery voltage at grid heater relays and measures heater element resistance and current draw during cold soak.
• Clamp-on DC ammeter — Confirms the intake air grid heater actually draws current when commanded on a cold start.
• Infrared thermometer — Checks intake manifold heating and verifies engine reaches normal operating temperature after cold start.

Rough cost and difficulty

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 rail pressure is low, fuel supply is suspect, or injector replacement is needed, have a diesel technician perform fuel system tests and injector service. Do not attempt high-pressure fuel system service without proper training and safety 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

• P2609 — Intake Air Heater System Performance
• P0263 — Cylinder 1 Contribution/Balance Fault
• P0128 — Coolant Thermostat Below Regulating Temperature

Sources