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.
NOTE: Whether your Ford Explorer, Aerostar, or Mercury Mountaineer is a pre-OBD2 vehicle (with no onboard misfire diagnostics) or an OBD2 vehicle, the information in this tutorial applies to both types of vehicles.
Contents of this tutorial:
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You can find this tutorial in Spanish here: Cómo Probar Fallas En Cilindro (4.0L V6 Ford) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Aerostar: 1991, 1992, 1993, 1994, 1995, 1996, 1997.
- 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
- 4.0L V6 Mercury Mountaineer: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
ENGINE NO-START DIAGNOSTICS: How To Troubleshoot A No-Start Problem (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
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:
- The check engine light (CEL) is on.
- The check engine light flashes on and off when the engine is experiencing the misfire.
- One or more misfire codes (P0300-P0306) will be stored in your Ford's PCM memory.
- P0300 Random Cylinder Misfire.
- P0301 Cylinder #1 Misfire.
- P0302 Cylinder #2 Misfire.
- P0303 Cylinder #3 Misfire.
- P0304 Cylinder #4 Misfire.
- P0305 Cylinder #5 Misfire.
- P0306 Cylinder #6 Misfire.
- Sometimes, even tho' the engine is suffering a bonafide misfire, no misfire codes are registered and no check engine light (CEL) comes on.
- Lack of power upon acceleration.
- Smell of unburned gas exiting the tail pipe.
- Rough idle and may stall.
- Cranks but does not start.
- Will not pass the emissions tests.
- 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:
- Fuel.
- Spark.
- 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:
- Bad ignition coil.
- Bad spark plugs.
- 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.
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.
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:
- Bad fuel injector.
- Broken fuel injector connector (this is a very, very common problem).
- Electrical short in the fuel injector wires that are keeping the fuel injector pulse signal from reaching the fuel injector.
- This is usually the result of human error and after a major mechanical repair where the wiring harness was damaged.
- Bad fuel injection computer not pulsing the fuel injector (this is a very rare condition, but it happens).
- Bad fuel pump.
How Can I Test The Misfire?
Troubleshooting a misfire condition on your 4.0L equipped Ford Explorer (Ranger, Aerostar, or Mountaineer) can seem quite intimidating but it shouldn't be. As I mentioned earlier, the key to diagnosing the cause of the misfire condition (or code) is to find out what is missing from the misfiring cylinder.
The misfiring cylinder will have one of three things missing: spark, fuel, or air (compression). It's as simple as this.
In this section, I'll show you a specific diagnostic strategy that'll help you find the component that's causing the problem.
STEP 1: Identify the dead cylinder. This is the most important first test that will save you a lot of time and frustration down the road by narrowing down and focusing your misfire diagnostic.
On the OBD II equipped Ford Explorer, Aerostar, and Mercury Mountaineer, identifying the dead (misfiring) cylinder usually means connecting a scan tool and checking for misfire codes (P0301, P0302, P0303, P0304, P0305, or P0306). Then by matching the misfire code to its engine cylinder using an illustration of the engine cylinders.
On OBD1 vehicles (1995 or older), since there are no onboard misfire diagnostics, you'll need to perform a cylinder balance test to determine which cylinder is the dead one.
STEP 2: Test the ignition system. Once you know which of the 6 cylinders is the one that's dead, the next step is to make sure that cylinder is getting spark (with a spark tester).
This is what I recommend:
- Check that the spark plug wire is delivering spark with a spark tester.
- Testing for spark with a spark tester is the most important first test since you'll know right away if the misfire is due to a lack of spark.
- If you got spark from your spark tester (from the cylinder the misfire code is accusing of misfiring), the next step is to remove the spark plug or spark plugs (of the affected cylinders) and check them for wear and tear, carbon tracks, anti-freeze, etc.
- If you get no spark, then the next step is to check for spark directly on the ignition coil pack tower.
- You can find all of these ignition system tests here:
- How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L) (at: easyautodiagnostics.com).
STEP 3: Test the compression of the engine cylinder(s). Other tests that should be done, if the ignition system and fuel injectors check out OK are:
- Engine compression test.
- Checking for vacuum leaks.
- You can find the engine compression test here:
STEP 4: Test the fuel injector(s). If the "dead" cylinder is getting spark and has good compression, then your next step is to eliminate its fuel injector as the cause of the cylinder misfire.
This is what I recommend you do:
- Resistance test the fuel injector that belongs to the ‘dead’ cylinder (along with at least 2 others).
- Do a Noid light test of each one to make sure the PCM is pulsing them (activating them).
- You can find the fuel injector tests here:
The above list of steps may seem/sound like troubleshooting a misfire is a complicated thing but it really isn't. Remember, the key to finding the exact cause of the misfire is to find out what is missing from the misfiring cylinder and depending on your wrenching experience, this is something you can do yourself.
What Tools Do I Need To Test The Misfire Code(s)?
Finding the exact cause of the misfire codes or misfire condition is possible with the proper tools. Without them, you won't be able to diagnose/troubleshoot those issues on your 4.0L equipped Explorer (Ranger, Aerostar, or Mountaineer).
Depending on what the root cause of the misfire is, you may need several tools. Most of these you can buy online, none of these will break the bank and I'll make some recommendations on them. Here's a guide to some of the basic tools that can be and are used:
- Ignition System Tests:
- Spark tester.
- Multimeter.
- Test light.
- Fuel System Tests:
- Noid light.
- Fuel pressure gauge.
- Multimeter.
- Engine Mechanical Tests:
- Compression tester.
Now of course, you'll also need basic hand tools like: screw-drivers, ratchet wrenches, sockets, etc. You'll also need a generic scan tool to retrieve the diagnostic trouble codes (DTCs) from the computer's memory.
Keep in mind that using the right tool for the job will save you time, frustration, and /or keep you from damaging the component that you're testing.
More Ford 4.0L Tutorials
You can find a complete list of 4.0L V6 Ford Explorer, Aerostar, and Mercury Mountaineer troubleshooting tutorials in this index:
Here's a sample of the tutorials you'll find in the index:
- How To Test The Fuel Pump (1993-2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Troubleshoot A No-Start Problem (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Fuel Injectors (1991-1996 4.0L Ford Explorer And Aerostar).
- How To Test The Ford EGR Valve, EGR Vacuum Solenoid, DPFE Sensor (at: easyautodiagnostics.com).
- How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L) (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
