How To Diagnose Misfire Codes (Ford 3.0L, 3.8L)

Troubleshooting a misfire (also known as an engine miss) can be a challenge since so many different things can cause one.

Although a misfire will usually light up the check engine light, on your OBD II equipped Ford vehicle, this is not an absolute truth, and this is what makes finding the root cause of the misfire difficult.

Well, in this article, I'm gonna' offer you my take on how to diagnose this type of problem.

En Español You can find this tutorial in Spanish here: Cómo Probar Fallas en Cilindro (Ford 3.0L, 3.8L) (at:

What is a Misfire Condition?

In plain English, a misfire condition, means that the engine in your Ford 3.0L, 3.8L vehicle is not running on all 6 cylinders.

When this happens, you'll notice some of the following consequences (symptoms):

  1. The check engine light will be on to let you know that you're not imagining the engine is running rough or missing.
  2. One or more misfire codes (P0300-P0308) will be stored in your pick up's PCM memory.
    1. P0300 Random Cylinder Misfire.
    2. P0301 Cylinder #1 Misfire.
    3. P0302 Cylinder #2 Misfire.
    4. P0303 Cylinder #3 Misfire.
    5. P0304 Cylinder #4 Misfire.
    6. P0305 Cylinder #5 Misfire.
    7. P0306 Cylinder #6 Misfire.
  3. Sometimes, even tho' the engine is suffering a bona-fide misfire, no misfire codes are registered in the computer's memory and/or no check engine light (CEL) on.
  4. Lack of power upon acceleration.
  5. Smell of unburned gas exiting the tail pipe.
  6. Rough idle and may stall.
  7. Cranks but does not start.
  8. Will not pass the emissions tests.
  9. Bad gas mileage.

Normally, the PCM (Powertrain Control Module = Fuel Injection Computer) will identify which cylinder is the one misfiring. What the PCM doesn't tell you is what exactly has failed.

What Causes A Misfire Condition?

Since the PCM doesn't tell you exactly what went wrong, it becomes important to know what are the common causes of a misfire condition.

The misfire cause will lie in one of three areas and they are:

Ignition System: Each engine cylinder needs spark to ignite the Fuel and the Ignition System is the one responsible for the production and delivery of said spark.

In my opinion, around 90% of the time, a fault in the Ignition System is usually behind a misfire. An Ignition System misfire is usually caused by:

  1. BAD spark plugs.
  2. BAD spark plug wires.
    1. What usually happens is one or two spark plug wires go BAD and stop transmitting spark to the spark plug.
    2. This is something that can easily be tested and you can find the tutorial here:
      1. How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L)
  3. BAD coil pack.
    1. What usually happens is one or two towers of the coil pack stop creating spark.
    2. This can easy be tested and you can find the tutorial here:
      1. How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L)

Fuel System: If fuel is missing from any one cylinder... the vehicle will have a miss that will light up the check engine light with a misfire code.

The most common type of failure is:

  1. BAD fuel injector.
    1. This happens when the fuel injector has reached its life's limit and fries internally.
    2. You can perform a fuel injector resistance test to see if it has fried internally and you can find the tutorial here:
      1. How To Test The Fuel Injectors (Ford 3.0L, 3.8L).
    3. Or the fuel injector becomes clogged.

Engine Mechanical Condition: If the engine cylinder is getting both fuel and spark but the pistons rings and/or valves are too worn out... you'll have a bona-fide misfire on your hands.

  1. You can find the compression test here:
    1. How To Do And Interpret an Engine Compression Test (Ford 3.0L, 3.8L).

Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.