How to Test the Fuel Injectors
(Ford 4.0L)

1994 Ford Explorer 4.0L (How to Test the Fuel Injector)

Testing the fuel injectors on the 4.0L Ford fuel injected engines in the Explorer, Aerostar, Ranger and Mercury Mountaineer can be quite a challenge since three of them are under the intake manifold plenum. But once you get to them, the test itself (of the fuel injectors) is very easy.

Also, when they fail, they usually don't leave a specific fuel injector diagnostic trouble code (DTC). What I have experienced is the PCM only registering a misfire code (P0300, P0301, P0302, P0303, P0304, P0305, P0306) or nothing at all.

Although fuel injectors don't go BAD very often, they also don't last for ever and this little article will show you how to test them with a multimeter. No scan tool required.

Here are the contents of this article at a quick glance:

  1. Important Tips.
  2. The Fuel Injector Test.
  3. Which Fuel Injector Do I Test First? or Do I Test all of Them?
  4. A Fuel Injector Troubleshooting Strategy.
  5. More 4.0L Test Articles.

En Español You can find this tutorial in Spanish here: Cómo Probar los Inyectores de Combustible (Ford 4.0L) (at:

Tools You'll Need:

  1. Multimeter
  2. Pen and Paper

Important Tips

If you do have to remove the intake manifold's plenum, to get to the three fuel injectors underneath it, you need to keep in mind several important things:

  1. Be careful that no foreign object, like a bolt, a nut, or any metal piece/part, falls into the open manifold port runners.
  2. Once the plenum has been removed, place a clean rag or rags on the open intake runners. This will keep things from falling into them.
  3. As you're removing bolts, nuts and stuff from the intake plenum to remove it, place them in a container and away from the engine compartment..

Anything falls into the open intake runners, without you knowing, and the engine is started... you are going to be in a big world of hurt! Since it may require the removal of the cylinder head to retrieve the part that fell in. So be alert and be careful.

Removing the plenum is not an out of this world thing that no one can do... it's done around the country in many shops without complications or unhappy endings by simply following some precautions.

The Fuel Injector Test

Testing the Resistance of the Fuel Injector With a Multimeter (Ford 4.0L)

If possible, you need to test the fuel injectors with a warm engine (not hot, but warm). So, if you're starting out with a cold engine in your 4.0L Ford Explorer (or 4.0L Aerostar, 4.0L Ranger, etc.) you need to start and run the vehicle for about 20 minutes. If the engine is too hot, let it cool down for about an hour.

If your Explorer (Ranger, Aerostar, or Mountaineer) doesn't start (cause you've already taken it apart) don't worry about warming up the enigne... you can still test the fuel injectors per the instructions in this article.

I'm sure that you are already aware that the upper intake manifold plenum has to be removed to access half of the fuel injectors. If you haven't removed it yet, take a look at the following section: Important Tips. Alright, enough talking, here's the test:

  1. 1

    After removing the intake manifold's plenum and placing clean rags in the open intake runners, disconnect all six fuel injectors from their electrical connectors.

    You don't have to test them all, but if you do have the intake manifold plenum off of the intake manifold, you might as well.

  2. 2

    Place your multimeter in Ohms mode and probe the metal male spade terminals inside the fuel injector with the multimeter leads. What you're doing is measuring the resistance of the fuel injector.

  3. 3

    Measure the resistance of the same fuel injectors several times so that you can be sure of your multimeter's Ohms result. When done, write this Ohms value down and record the number of the cylinder that fuel injector belongs to, too.

  4. 4

    The fuel injector resistance will be about 11-18 Ohms. If on your Ford Explorer or Aerostar or Ranger this value is different... don't worry about this too much for now. OK, repeat test steps on the remaining fuel injectors.

Alright, with the resistance values of all six fuel injectors on paper, let's interpret your results. Choose from one of the following CASES:

CASE 1: All of the multimeter resistance values are nearly identical: This result tells you that the fuel injectors are good and are not causing the misfire code (P0300, P0301, P0302, P0303, P0304, P0305, P0306) or Rough Idle Condition.

If you did have one BAD fuel injector in the whole bunch... you would get a drastically different Ohms value in one of them. For example.. if 5 of the 6 measured 11.5 to 11.9 Ohms and the sixth one measured 4 Ohms. Well, the one measuring the 4 Ohms is the one that's fried.

CASE 2: One of the fuel injector multimeter resistance values is radically different: Retest that one fuel injector once again. If the fuel injector registers the same Ohms value as before... then that fuel injector is BAD.

Ford Vehicles:

  • Aerostar 4.0L
    • 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003

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