How to determine if fuel injectors are leaking

Written by jack hathcoat
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How to determine if fuel injectors are leaking
Routine under-the-bonnet inspections for fuel leaks is important. (under the bonnet image by Lucy Cherniak from

Fuel injector leaks are often hard to diagnose because the leak is only active when the car is running and the fuel system is pressurised. The leaks are also small and do not create large puddles of gasoline lying beneath the car. Knowing how to identify a fuel injector leak is important for safe driving. Leaking injectors are fire hazards because they spray or drip fuel in the engine bay and are often near hot exhaust components.

Skill level:


  1. 1

    Raise the bonnet of the car and see if any fuel odours are present. Under normal conditions, there are no gasoline odours in the engine compartment. This is the first indicator of a fuel leak.

  2. 2

    Locate the fuel injector rail and look at each of the fuel injectors. The rail is usually at the top-back of the engine and is located immediately behind the engine valve cover. This arrangement is normal for most front-wheel drive, four-cylinder cars. V-6 and V-8 engines have two injector rails and the injectors are on both sides of the top of the engine.

  3. 3

    Inspect each injector. The injectors are tubular, have wires plugged into them, and are found under a common fuel rail that holds each of the injectors in place. There is one injector per cylinder. On the top and bottom of the injectors are special O-rings. Turn the key on but do not start the car. Turning the key on will pressurise the injector rail and starting the car is not necessary.

  4. 4

    Look at each injector. If the O-ring seal is defective, a fine mist of gasoline sprays out of the seal. In some cases there is only seepage, so feel around each injector and see if there is wet gasoline leaking from around the seal. If there is a leak, immediately turn the key off and make the necessary repairs.

Tips and warnings

  • A good indicator of a fuel injector leak is a sudden and dramatic drop in fuel mileage. This is especially true if you happen to notice a faint odour of fuel at the same time. Injectors also fail electrically. When this happen, the engine stumbles and misses from lack of fuel supply to one of the cylinders.
  • Injectors can fail electrically. This means they will not pulse or spray fuel into the engine. In these cases, the car will misfire and run poorly.

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