How To Test The Fuel Injectors (1990, 1991, 1993, 1994, 1995, 1996 4.0L Ford Ranger And Mazda B4000)

The fuel injectors are one of the most challenging components to test on your Ford Ranger or Mazda B4000's 4.0L V6 engine. What makes the test a challenge is the fact that three of them are underneath the intake manifold plenum.

This tutorial will present the process I've used to narrow down the problem to a faulty fuel injector. I'm confident it'll help you in your troubleshooting efforts.

NOTE: You can find the 1991-1996 4.0L Ford Explorer (Aerostar) fuel injector test here:

APPLIES TO: This tutorial applies to the following vehicles:

  • 4.0L V6 Ford Ranger: 1990, 1991, 1992, 1993, 1994, 1995, 1996.
  • 4.0L V6 Mazda B4000: 1994, 1995, 1996.

You can find the 1997-2000 4.0L Ford Ranger (Mazda B4000) fuel injector test tutorial here:

Fuel Injector Resistance Specifications

1990-1992
Engine Ohms
4.0L Ranger 13-16 Ohms
1993-1996
Engine Ohms
4.0L Ranger 11-18 Ohms
4.0L B4000 11-18 Ohms

The Fuel Injector Resistance Test

Testing The Resistance Of The Fuel Injector With A Multimeter (1990, 1991, 1993, 1994, 1995, 1996 4.0L Ford Ranger And Mazda B4000)

This section outlines the fuel injector resistance test. If you haven't narrowed down the dead cylinder yet, then please take a look at this section:

You'll need a multimeter to test the fuel injector resistance. If you don't have one, check out my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.

These are the test steps:

  1. 1

    Removing the intake manifold's plenum.

    NOTE: Place clean rags in the open intake runners.

    See this section: Important Precautions When Removing The Intake Manifold Plenum.

  2. 2

    Disconnect all six fuel injectors from their electrical connectors.

  3. 3

    Place your multimeter in Ohms mode.

  4. 4

    Probe the metal male spade terminals inside the fuel injector with the multimeter test leads.

  5. 5

    Your multimeter should register the indicated resistance specification.

    NOTE: You can find the resistance specification here: Fuel Injector Resistance Specifications

  6. 6

    Repeat test steps on the remaining fuel injectors.

Let's interpret your test results:

CASE 1: All resistance values were within specification. This is the correct and expected test result and it tells you that the fuel injectors do not have an internal short-circuit or open-circuit problem.

CASE 2: One of the fuel injector resistance values is not within specification. Retest that one fuel injector once again.

If the fuel injector registers the same Ohms value as before, the fuel injector is bad and needs replacement.

Fuel Injector Troubleshooting Strategy

Diagnosing a bad fuel injector on your 4.0L Ford Ranger or Mazda B4000 can be a challenge since you'll need to remove the intake manifold's plenum to test 3 of them.

Fortunately, there is a troubleshooting strategy that may help you avoid removing the plenum. Not only that, it'll help you get to the bottom of the misfire or rough idle issue without wasting time and money.

I've often used this diagnostic strategy to find the bad fuel injector. So I'm confident it'll help you too.

PART 1: Identify The Dead Cylinder.

Identifying the 'dead' cylinder is the critical first step of this fuel injector diagnostic strategy.

You can pinpoint the 'dead' cylinder by performing a cylinder balance test.

The biggest benefit of the cylinder balance test is that it'll help determine if the 'dead' cylinder's fuel injector is located underneath the intake manifold plenum.

NOTE: Even if your Ford Ranger (Mazda B4000) is OBD II equipped and you've got a cylinder misfire diagnostic trouble code, do the cylinder balance test to make sure you've identified the correct cylinder.

PART 2: Make Sure The Dead Cylinder Is Getting Spark.

After you've identified the 'dead' cylinder, your next step is to make sure its spark plug wire delivers spark to its spark plug.

You must perform the spark test with a dedicated spark tester. Any spark test method that does not involve a spark tester will only have you chasing ghosts (and wasting time and money).

You'll also need to:

  • Check the spark plug for any apparent damage.
  • Check the spark plug for excessive wear.
  • Check the spark plug electrodes are not blocked with carbon.
  • Check the spark plug's air gap.
  • Check the spark plug, and the spark plug wire does not have carbon tracks.
  • Check the spark plug wire isn't missing the metal connector that connects to the spark plug.

If the 'dead' cylinder isn't getting spark, you've found the source of the misfire (rough idle), you don't need to test the fuel injectors.

If the 'dead' cylinder is getting spark and the spark plug and spark plug wire are OK, the next step is checking the 'dead' cylinder's compression.

PART 3: Make Sure The Dead Cylinder Has Compression.

If the ignition system is not causing the 'dead' cylinder issue, your next step is to make sure the 'dead' cylinder produces enough compression to start and complete the air/fuel mixture's combustion process.

Now, you don't need to test all six cylinders. But for the accuracy of your diagnostic, you should test all six.

You can find the compression test explained in detail here:

If the 'dead' cylinder's compression value is 15% lower than the highest compression reading your compression test has given you, then you've found the cause of the misfire (rough idle) issue, and you don't need to test the fuel injectors.

If the 'dead' cylinder's compression value is within a normal range, your next step is to test the fuel injectors.

PART 4: Test The Fuel Injectors.

If you've reached this point, you now know that:

  • The ignition system is not behind the problem.
  • The cylinder's compression is not behind the problem.

If the 'dead' cylinder's fuel injector is underneath the intake manifold plenum, you can now make plans to remove it and resistance test the fuel injector.

Ford Vehicles:

  • Ranger 4.0L
    • 1990, 1991, 1992, 1993, 1994, 1995, 1996

Mazda Vehicles:

  • B4000 4.0L
    • 1994, 1995, 1996