White or light grey smoke during and after cold starting. Unburned diesel vapor from a combustion chamber that is too cold to ignite fuel properly. Usually resolves once the engine warms up. If it persists, glow plugs, injection timing, or compression may be at fault.

What you'll see

White or light grey smoke from the exhaust during and immediately after cold starting. The smoke has a raw diesel smell. The engine may run rough for the first few minutes. In cold weather, some white smoke on startup is normal and clears as the engine warms up. But if it continues for more than 5-10 minutes, or if it happens in warm weather too, something is wrong.
Before you assume this is the problem

A small amount of white smoke on cold startup that clears within a few minutes is normal diesel behavior, especially in cold weather. Only investigate if it persists beyond warm-up or happens in warm conditions. Sweet-smelling white smoke (different from diesel smell) can indicate coolant leaking into the combustion chamber via a head gasket failure -- that is a separate and more serious issue.

See all causes of excessive smoke / poor engine power →

How to diagnose

  1. Test the glow plugs

    Faulty glow plugs cannot preheat the combustion chamber sufficiently, leading to incomplete combustion and white smoke on cold starts. Test each glow plug with a multimeter -- resistance should be 0.5-2 ohms. An open circuit means the plug is burned out. Even one bad plug on a 3 or 4 cylinder engine can cause white smoke.
    Result: 0.5-2 ohms = OK. Open circuit = replace.

  2. Check injection timing

    Late injection timing causes white smoke because fuel is injected after the optimal compression point, resulting in incomplete combustion. On mechanical engines, timing can drift if the injection pump coupling slips or wears. On electronic engines, the ECU controls timing and a fault code may be stored.
    Result: Timing within spec = OK. Late timing = adjust.

  3. Check engine compression

    Low compression (from worn rings, valves, or a blown head gasket) reduces the temperature in the cylinder at the end of the compression stroke. If the temperature is too low, diesel does not ignite reliably. A compression test on each cylinder will reveal low or uneven compression. This is a more serious finding.
    Result: Even compression across all cylinders at spec = OK. Low/uneven = internal engine wear.

How to fix it

  1. Replace glow plugs

    Replace any burned-out glow plugs. Use anti-seize on the threads. Consider replacing all of them if the engine has high hours -- if one failed, the others are likely close behind. Also check the glow plug relay and timer circuit.

  2. Correct injection timing

    Injection timing adjustment is a specialist job requiring proper tools (timing dial gauge, lock pin). If you suspect timing has drifted, have a diesel mechanic check and set it. Do not attempt to adjust by ear -- even 1-2 degrees off can cause problems.

Common mistakes

Do not ignore persistent white smoke -- it means fuel is not burning properly, which washes oil off the cylinder walls and accelerates ring and liner wear. Also: do not confuse white smoke (unburned diesel) with steam (coolant leak). Steam has no diesel smell and dissipates quickly in the air. White diesel smoke lingers and smells like raw fuel.

Parts & tools

Multimeter for glow plug testing. Replacement glow plugs. Glow plug socket. For timing and compression work: diesel mechanic with proper tools.

Review safety precautions before starting →

Safety

White diesel smoke is unburned fuel vapor -- flammable and toxic. Ensure adequate ventilation.

This issue can also cause

  • Engine Won't Start Diesel engine won't crank or won't fire—usually electrical, fuel system, or glow plug related.