How to Test the Fuel Injectors
(Ford 4.0L)

Which Fuel Injector do I Test First?
or Do I Test all of Them?

You don't have to test all fuel injectors. For example, you might have Cylinder #4 Misfiring or dead and you want to just test that particular fuel injector... well that's OK.

My suggestion is to test the two adjacent fuel injectors so that you can have at least two other resistance (Ohms) value to compare the one you suspect as BAD.

If you don't have a specific misfire code to go on... well that's OK too... just test all of the fuel injectors.

Now, let's say that the one you want to test is under the intake manifold plenum and you need to remove it to get to it... well, in this situation you can also only test the one you think is BAD and at least two others, instead of all of them.

A Fuel Injector Troubleshooting Strategy

Diagnosing a BAD fuel injector on your 4.0L Ford can be a challenge... since you need to remove the intake manifold's plenum to test 3 of them. So, in this section, I'm gonna' offer you my fuel injector troubleshooting strategy so that you can get to the bottom of the problem without wasting time and money.

These are the steps I take:

  1. Eliminate the Ignition System as the root cause of the ‘dead' cylinder.
    1. I want to emphasize this to you: It's very important to eliminate the ignition system from the get-go.
    2. What I'm looking for here... is to confirm that the spark plug is getting spark and I do this with a dedicated spark tester (like an HEI Spark Tester).
      1. The following tutorial will help you test the ignition system: How to Test the Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L).
  2. Next, I remove the spark plugs and visually inspect them for damage or excessive wear and tear.
    1. What I'm checking for are for cracks and/or carbon tracks and/or oil filling up the spark plug well.
    2. I also remove and visually inspect the spark plug cable and the ignition coil pack tower (that the spark plug wire is connected to) for visible rust, corrosion, wear and tear, etc.
  3. If everything above checks out OK... I then do an engine compression test on the ‘dead’ cylinder and at least 2 others (to be able to compare compression values).
    1. low cylinder compression is so often overlooked as a cause of a engine miss (misfire, ‘dead’ cylinder, etc).
    2. What I'm looking for is to see if any one cylinder is too worn out. If it's too worn will always have 15% lower compression that the highest compression reading of the four.
      1. The following tutorial will help you test engine compression: How to Test Compression (Ford 4.0L).
  4. Check fuel injector resistance.
    1. So, if you've reached this point, it's because you now know that the ignition system is not the cause of the engine miss (misfire, ‘dead’ cylinder, etc.), you've also eliminated the cylinder's compression and so... you know can start testing the fuel injector.

Remember, the most important step... is to find the ‘dead’ cylinder first! Finding the misfiring cylinder is half the battle to correctly diagnosing the engine miss (misfire, ‘dead’ cylinder, etc.) that's afflicting your 4.0L Ford Explorer (Ranger, Aerostar or Mountaineer)!

More Ford 4.0L Test Articles

Here are links to other articles that may be of help in troubleshooting your Ford 4.0L equipped vehicle that I've written. To see the articles in this Web Site, click here: Ford 4.0L Index of Articles.

Here's a sample of the tutorials you'll find in the index:

  1. How to Test Compression (Ford 4.0L).
  2. How to Test the Thermostat (Ford 4.0L).
  3. How to Test the Ford V6 Ignition Coil Pack (at:
  4. How to Test the MAF Sensor (at:
  5. How to Test the Throttle Position Sensor (TPS) (at:

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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