How To Diagnose Misfire Codes P0300-P0308 (Ford 4.6L, 5.4L)

Testing a misfire code (P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308) can be quite a challenge, since so many different things can cause a misfire on your 4.6l, 5.4L Ford vehicle.

With this article, I'm gonna' help you demystify the whole process of testing a misfire condition on your Ford car, SUV or pick up with either a 4.6L or 5.4L V8 by showing you a very simple (but effective) diagnostic strategy that you can use (to troubleshoot the misfire).

More importantly, at the end of the article, I'll tell you where to find the step-by-step misfire test articles.

You can find this tutorial in Spanish here: Cómo Probar Una Falla En Cilindro (4.6L, 5.4L Ford V8) (at: autotecnico-online.com).

What Is A Misfire Condition?

A misfire condition, in plain English, describes an engine that is not running on all cylinders in your Ford F150 (or F250-350, Expedition, Excursion, Crown Victoria, Thunderbird, or Mercury Grand Marquis, Cougar, etc).

Therefore, if one cylinder or two cut out, the engine will not output 100% of its power and you're gonna' feel it (and smell it too, in the form of a rich exhaust gas smell coming out of the tail pipe).

Here are some of the most common symptoms of a misfire condition:

1. The check engine light will be on.
2. One or more misfire codes (P0300, P0301, P0302, P0303, P0304) will be stored in the car or pickup's computer's 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.
   • P0307 Cylinder #7 Misfire.
   • P0308 Cylinder #8 Misfire.
3. Misfire is present, but 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 (smog check).
9. Bad gas mileage.

Having worked around cars (as an automotive tech) for 20 years now and taking into account that most of you reading this article are NOT professional automotive techs, I can tell you that testing a misfire condition is not hard.

In the next section, we'll explore some of the common areas these misfires live in and in the process we'll find out what causes a misfire condition. Let's read on and find out.

What Causes A Misfire Condition?

As you may already know, each cylinder needs 3 very specific things to produce power and they are:

1. Air
2. Fuel
3. Spark

If any one of these components is missing in any one engine cylinder, your 4.6L, 5.4L engine cylinder will misfire. So then, the fault could lie in:

1. The ignition system, which is responsible for creating and delivering spark.
2. The fuel system, which is responsible for supplying each engine cylinder with fuel.
3. Inside the engine, specifically piston rings, cylinder head valves, intake manifold gaskets (these are responsible for the air part of the air, fuel, and spark mixture).

In most cases, it's usually a lack of spark that's to blame for the misfire, but not always. Let's take a brief look at the different things that can cause your 4.6L or 5.4L Ford engine to misfire:

Ignition System: If any one engine cylinder is not getting spark, then you'll have a bonafide misfire on your hands.

The following ignition system components could cause a misfire when they fail:

1. Bad COP (Coil-On-Plug) ignition coil.
2. Broken COP ignition coil connector (this is a very, very common problem).
3. Bad coil pack (if your engine has a coil pack ignition system).
4. Bad spark plug or spark plugs
5. Bad ignition coil boot (on COP ignition coil systems).
6. Carbon tracks on the spark plug and spark plug boot.
7. Bad spark plug wires (if your engine has a coil pack ignition system).

The good news is that all of the components that make up the ignition system can be tested, and I'll show you how.

Fuel System: If fuel is missing from any one specific engine cylinder, it will misfire.

The following fuel system components could cause a misfire when they fail:

1. Bad fuel injectors.
2. Electrical short in the fuel injector wires that are keeping the fuel injector pulse signal from reaching the fuel injector.
3. Broken fuel injector connector (this is a very, very common problem).
4. Bad PCM (this is very rare, but it does happen).

Testing the fuel system components, that could cause a misfire can also be easily tested.

Engine Mechanical Condition: This is one of the most overlooked conditions when testing for a misfire.

The following engine mechanical problems could cause a misfire:

1. An engine cylinder that is not producing enough compression.
   • Even it has the spark and fuel, but the compression is low, that cylinder will misfire and set a misfire code.
2. Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.
   • Ford 4.6 and 5.4L intake manifold gaskets are made of plastic and are notorious for causing engine misfire conditions when they fail and start leaking anti-freeze into the spark plug tubes (spark plug wells).