How to Test the Fuel Injectors (5.0L Engine, 5.8L Engine)

Testing the fuel injectors on the fuel injected 5.0L engine and 5.8L engine can be quite a challenge since half of the 8 fuel injectors are under the intake manifold plenum.

In this tutorial, I'm gonna' show you a specific diagnostic strategy you can use to find out if you do indeed have a bad fuel injector on your hands or not.

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

  1. Symptoms of a BAD Fuel Injector.
  2. What Tools do I Need to Test the Fuel Injectors.
  3. Start Here: Diagnostic Strategy.
  4. The Fuel Injector Test.
  5. Interpreting the Results of the Fuel Injector Test.
  6. Important Tips

You can find this tutorial in Spanish here: Cómo Probar los Inyectores de Combustible (Ford 5.0L, 5.8L) (at: autotecnico-online.com).

Symptoms of a BAD Fuel Injector

The most common symptom of a BAD fuel injector is an engine miss at idle or when you accelerate the vehicle. Other symptoms include but not limited to:

  1. Lack of power.
  2. BAD gas mileage.
  3. Check engine light (CEL) is on.

Now, to go into more specifics of how a fuel injector can fail on your 5.0L engine or 5.8L engine:

  1. The fuel injector fries internally and won't spray any fuel.
  2. The fuel injector gets clogged with deposits and won't spray enough fuel or in the proper spray pattern.
  3. The fuel injector doesn't activate (spray) because it's missing power
  4. The fuel injector doesn't activate because it does have power but the Fuel Injection Computer doesn't activate it.
  5. PCM goes bad and activates the fuel injector as soon as the Key is turned to the Run position.

No matter what the root cause, a BAD fuel injector will cause the engine to idle rough and miss when you accelerate it down the road.

What Tools do I Need to Test the Fuel Injectors?

To do a fuel injector resistance test, you don't need a whole lot of stuff, since the most important tool you'll need is a multimeter (which can either be an analog multimeter or a digital multimeter).

Here's a basic list of tools you'll need:

  1. A multimeter.
    1. You'll only be using the multimeter to check resistance (Ohms).
    2. If you need to upgrade or buy a multimeter, check out my recommendation: Buying a Digital Multimeter for Automotive Diagnostic Testing (found at: easyautodiagnostics.com).
  2. OBD I scan tool or code reader.
    1. To actually test the fuel injectors, you don't need a scan tool (since a scan tool can't dynamically test the fuel injectors). But, having one makes the whole process easier... since you're able to retrieve any diagnostic trouble codes (DTCs) stored in the PCM memory.
  3. Hand Tools.
    1. Since half of the fuel injectors are underneath the intake manifold plenum, you'll need hand tools like: socket wrench, extensions, sockets, etc. to remove it.
  4. Pen and Paper to write down your fuel injector resistance test results.

Start Here: Diagnostic Strategy

To avoid replacing parts that your Ford 5.0L engine or 5.8L engine doesn't need (or removing the intake plenum when you don't have to), you need a diagnostic strategy to find the BAD fuel injector.

So, if you don't know where to begin (to find the BAD fuel injector) or just need a little guidance, here's my own fuel injector troubleshooting strategy:

  1. Find out which cylinder is the one that's ‘dead’ first.
    1. A BAD fuel injector will cause the engine to miss (misfire). The cylinder that's misfiring is considered to be ‘dead’.
    2. To find this ‘dead’ cylinder, you need to do a cylinder balance test.
    3. Locating the ‘dead’ cylinder will save you a ton of diagnostic/troubleshooting time, not to mention a lot of frustration.
    4. You can find the tutorial I've written on the cylinder balance test here: 5.0L, 5.8L Cylinder Balance Test.
    5. After locating the ‘dead’ cylinder, the next step is to eliminate the ignition system as the cause of the miss.
  2. Eliminate the ignition system.
    1. This means that you need to make sure that the ‘dead’ cylinder is getting spark and that the spark plug is OK too.
    2. The ignition system is the most common culprit behind 90% of misfires (engine miss), so it's best to eliminate it right from the get go.
    3. After confirming that the cylinder is getting spark, the next step would be to test that cylinder's compression.
  3. Compression test the ‘dead’ cylinder.
    1. You don't have to test all 8 cylinders... just testing the ‘dead’ cylinder plus two other (to compare your compression test results to) is enough to get an idea of the internal health of that cylinder.
    2. After confirming that the ‘dead’ cylinder has good compression, the next step is to test the fuel injector's resistance.
  4. Test the fuel injector.
    1. This is gonna' involve testing the fuel injector's internal resistance.
    2. You'll also need to make sure it's getting power and the PCM's switching (activation) signal.

I've used the above diagnostic strategy with a ton of success over many years working as a diagnostic tech and I think it'll help you too.