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How to Locate a Bad Diesel Injector

Updated July 19, 2017

Over time, diesel fuel injectors wear out and fail to close completely. When this happens they cause a leak-down, resulting in a reduction of fuel pressure across the fuel rail. This can cause a difficult- or no-start condition as well as a smoky exhaust. Too much fuel will also flood a cylinder, effectively reducing power. There are two ways to locate a bad injector without spending the money taking it to the dealer for a diagnosis.

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  1. Start the engine and allow it to reach normal operating temperature. Check each cylinder temperature by taking the temperature of each exhaust manifold runner as close to each cylinder as possible. The exhaust manifold or manifolds that are cooler than the rest are the ones with the faulty injectors.

  2. Check the temperature with the spray bottle of water if an infrared sensor is not available. Spray water on the exhaust manifold where it attaches to the head. A good cylinder will evaporate the water instantly. A bad cylinder will allow the water to soak the exhaust runner and the water will evaporate slowly.

  3. Raise the rpm of the engine if the exhaust manifold runners are to close to call. This will raise the temperature in the good cylinders and make for a more accurate deduction with the sensor or the water method.

  4. Remove the metal fuel line connection at the feed tube, which in inserted in the side of the cylinder head. The feed tube sends fuel into the base of the fuel injector, which is located under the valve cover where it extends down into the cylinder head. Use a wrench to loosen and remove the connector from the feed tube.

  5. Install the fuel feed block cap on the fuel line to shut off the fuel. Do not run the engine long with the fuel line disconnected or it will ruin the injector, which is now running dry. Start the engine. If it starts easier and the exhaust is cleaner, you have found the faulty injector. If the engine will not start at all, it is most likely due to a badly leaking fuel injector causing low fuel pressure. If the engine starts, it is an indication of a bad injector. You must do this to each cylinder, one at a time.

  6. Check for obvious fuel leaks on the exhaust manifold caused by a bad injector flooding a cylinder.

  7. Tip

    Thermal temp gun can be purchased at any local parts store for fairly cheap! Keep your fuel filter changed often to avoid diesel injector failure! Additive for diesel fuel do work and can help lubricate diesel injector!


    Never have anyone but qualified automotive or diesel engine technician work on your vehicle! Exhaust manifolds get extremely hot! Be careful not to touch them!

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Things You'll Need

  • Infrared heat sensor
  • Fuel injector feed port caps (special tool to block the fuel line to the feed tube to the injector once disconnected). You can purchase these at an auto parts store or the dealer.
  • Set of wrenches
  • Spray bottle of water (in place of an infrared temperature sensor, less accurate but effective)

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).

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