Worn or clogged fuel injectors spray fuel poorly -- dripping instead of atomizing, spraying off-center, or stuck partially open. This causes incomplete combustion, black smoke, power loss, and rough running. Often accompanied by increased fuel consumption.

What you'll see

Black smoke from the exhaust even with a clean air filter. The engine may run rough or misfire on one or more cylinders. Power is reduced. You may notice a distinct knocking or rattling from the engine that was not there before. Fuel consumption is higher than normal because the injectors are not atomizing fuel properly -- large droplets burn incompletely. The exhaust may have a raw diesel smell.
Before you assume this is the problem

Make sure the air filter is clean first -- a dirty filter causes the same symptoms and is much cheaper to fix. Also check fuel quality -- contaminated or old fuel can cause injector problems. See: Black Smoke from Clogged Air Filter.

See all causes of excessive smoke / poor engine power →

How to diagnose

  1. Identify the misfiring cylinder

    With the engine running, loosen the high-pressure fuel line at each injector one at a time (be careful -- high pressure!). If loosening a line causes a noticeable drop in engine speed and roughness, that cylinder was working. If loosening a line makes no difference, that cylinder was already not contributing -- its injector is likely faulty.
    Result: No change when disconnecting a cylinder = that injector is suspect.

  2. Check injector spray pattern

    Remove the suspect injector(s) and test them on a bench tester if available. A good injector produces a fine, cone-shaped mist at the correct opening pressure. A faulty injector drips, sprays to one side, or has a distorted pattern. The opening pressure should match the engine specification.
    Result: Fine mist at correct pressure = injector OK. Dripping, off-center, or wrong pressure = faulty.

How to fix it

  1. Have injectors serviced or replaced

    Injectors can be cleaned, re-tipped (new nozzle), and adjusted to correct opening pressure by an injection specialist. On older engines with mechanical injectors, this is straightforward and cost-effective. On newer common-rail engines, injectors are more complex and usually need replacement rather than repair. Always replace copper sealing washers when reinstalling injectors.

  2. Prevent future injector problems

    Use clean, quality diesel fuel. Change fuel filters on schedule. Drain the water separator regularly. Water in fuel is the number one cause of injector wear and damage. If the machine sits for long periods, use a fuel stabilizer.

Common mistakes

Do not swap injectors between cylinders to test -- different cylinders have different thermal characteristics and an injector working marginally in one cylinder may fail completely in another. Also: do not adjust injection pump timing without the proper tools and knowledge. Incorrect timing causes smoke, power loss, and engine damage.

Parts & tools

Replacement injectors or injector service kit. Copper sealing washers. Injector puller tool (some injectors seize in the head). Torque wrench for reinstallation. This is typically a diesel mechanic job.

Review safety precautions before starting →

Safety

High-pressure diesel from fuel injectors can penetrate skin and cause serious injury. Never put your hands near an injector while the engine is cranking or running. The injection pressure can be 200-2,000 bar depending on the system.

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