How To Test Misfire Codes (4.0L Ford Explorer, Ranger, Aerostar, Mercury Mountaineer)

One of the most common problems you're gonna' experience on your 4.0L equipped Ford is a misfire condition causing one or more of the following misfire codes: P0300, P0301, P0302, P0303, P0304, P0305, and/or P0306.

Although quite a few things can cause a misfire code (or codes) to pop up and light up the check engine light (CEL)... troubleshooting a misfire isn't all that hard.

That's right, it's not hard. There's a method to the madness of troubleshooting a misfire on your 4.0L Ford Explorer (Aerostar, Ranger or Mountaineer).

In this article, I'll go into the basics of what causes a misfire condition and I'll also offer you a basic diagnostic strategy to get to the bottom of what's causing the misfire problem.

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

  1. Symptoms of a Misfire Condition?
  2. What Causes A Misfire Condition?
  3. How Can I Test The Misfire?
  4. What Tools Do I Need To Test The Misfire Code(s)?
  5. More Ford 4.0L Tutorials.

En Español You can find this tutorial in Spanish here: Cómo Probar Fallas en Cilindro (4.0L V6 Ford) (at: autotecnico-online.com).

Symptoms Of A Misfire Condition?

The basic symptom of a misfire is a light to severe rough idle that's felt when the engine is idling and a light to severe engine miss (hesitation) when the engine is under load (for example: your Ford vehicle moving from a stop).

Here are other symptoms your 4.0L equipped Ford will experience:

  1. The check engine light (CEL) is on.
  2. The check engine light flashes on and off when the engine is experiencing the misfire.
  3. One or more misfire codes (P0300-P0306) will be stored in your Ford'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.
  4. Sometimes, even tho' the engine is suffering a bona-fide misfire, no misfire codes are registered and no check engine light (CEL) comes on.
  5. Lack of power upon acceleration.
  6. Smell of unburned gas exiting the tail pipe.
  7. Rough idle and may stall.
  8. Cranks but does not start.
  9. Will not pass the emissions tests.
  10. Bad gas mileage.

Although the misfire codes don't tell you what exactly is the cause of the misfire or rough idle condition... there is a way to find out exactly what is causing it.

Let's jump into the next heading and find out what are basic causes of a misfire.

What Causes A Misfire Condition?

An engine misfire condition occurs when one (or more) engine cylinders on are not producing power.

These ‘dead’ cylinders stop producing power because one of three things is missing:

  1. Fuel.
  2. Spark.
  3. Air.

So, the key to successfully diagnosing a misfire code/condition is to keep in mind that the cylinder that's misfiring (‘dead’), on your 4.0L Ford, is missing one of the above.

Let's take a look at some of the specific components, that when they fail, provoke a misfire or a rough idle condition.

Ignition System: The majority of misfire codes have their root cause in a failed ignition system component that isn't creating or delivering spark to the affected cylinder.

The usual suspects (that cause a misfire) are:

  1. BAD ignition coil.
  2. BAD spark plugs.
  3. Carbon tracks on the spark plug and spark plug boot.

Testing all of the ignition system components is not hard and it doesn't require expensive tools or expensive diagnostic equipment.

Fuel System: The Fuel System is responsible for the delivery of fuel. If fuel is missing from any one specific engine cylinder, it will misfire.

Fuel system problems could include some of the following:

  1. BAD fuel injectors.
  2. Broken fuel injector connector (this is a very, very common problem).
  3. Electrical short in the fuel injector wires that are keeping the fuel injector pulse signal from reaching the fuel injector.
    1. This is usually the result of human error and after a major mechanical repair where the wiring harness was damaged.
  4. BAD fuel injection computer not pulsing the fuel injector (this is a very rare condition, but it happens).
  5. BAD fuel pump.

Engine Mechanical Condition: The pistons and valves are the ones that draw air into the engine. Usually all cylinders wear out evenly... but every now and then, either thru' lack of maintenance or some mechanical problem, you'll have one or more wear out at an accelerated pace.

To make the long story short, those cylinders (with accelerated wear and tear) to produce a less than average compression value that will cause a misfire condition.

Other issues, that can not be overlooked are vacuum leaks.