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P0300 Random Misfire Diagnosis

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

A random misfire can be hard to diagnose.

A random misfire can be hard to diagnose.

A "P0300—Random Misfire" code can make your life miserable. Codes can go from one cylinder to another without a clear pattern. And they may appear for one or more different reasons:

  • Bad knock sensor
  • Bad spark plugs
  • Faulty ignition coil or pack
  • Leaking valves
  • Vacuum leak (throttle body, intake manifold, hose)
  • EGR valve leak
  • Failed oxygen sensor
  • Bad fuel injector
  • Blocked catalytic converter
  • Worn rings or cylinders
  • Too lean or rich air/fuel ratio conditions
  • Weak or worn fuel pump
  • Worn timing belt or chain

And the list goes on.

To this, you need to add the fact that since 1996, auto manufacturers have used different programming configurations to help the car's computer getter monitor and detect misfires.

This has resulted in false misfire problems, on some models, when a vehicle, for example, drives over irregular or rough roads. This has been solved on some models with the use of vehicle speed sensors that help the computer differentiate between a smooth and rough road.

Typically, a random misfire shows up because of an issue with the ignition or fuel system, the ignition timing, or engine compression.

Symptoms That May Show Up Along With P0300 Include:

  • Rough running
  • Hard starting
  • Jerking during acceleration
  • Decreased engine power
  • Check engine light (CEL) on or flashing
  • Poor fuel economy

Often, you'll find a weak spark or lean air-fuel mixture (too little fuel) behind a P0300 trouble code or intermittent misfire. Look for a potential faulty ignition coil or fuel injector.

Perhaps you want to start your diagnostic here. Still, the next sections will help you find the source of a random misfire.


1. Making the Most of Your Diagnostic Time

2. Fuel Related Misfires

3. Ignition Timing Misfires

4. Ignition System Misfires

5. Compression Related Misfires

6. Getting Professional Help

1. Making the Most of Your Diagnostic Time

Knowing where to start hunting for the source of that random misfire can be a daunting task in itself. However, here are some logical steps you can take to get yourself organized:

  • Make sure you're taking into account all the trouble codes stored in your car's computer memory, including pending codes. If there are codes other than a P0300, start diagnosing and try to fix those, if possible. This is because another problem in the system may have caused the computer to throw a random misfire code.
  • The systems and components listed in the next sections are the most common sources of random misfires. If you have neglected maintenance in any of those areas, concentrate your efforts there. Chances are they are the ones causing you trouble. For example, you may need to replace old spark plugs and wires, old fuel and air filter, clean and check the EGR valve. Catching up with your vehicle maintenance helps you fix and prevent other issues from showing up.
  • Have you noticed any engine problems in the recent past? Rough idle, stalling, fuel odors, hissing sounds coming from the engine; oil leaks around the valve cover. These and other signs of trouble can give you a clue as to where to look. The troubleshooting section of your vehicle repair manual can be of great help here.

    If you don't have the manual yet, get a relatively inexpensive aftermarket copy from Amazon. Haynes manuals come with the manufacturer suggested maintenance schedule, plus many step-by-step procedures for maintenance, troubleshooting and repairs projects you can do at home. So you'll recoup your investment soon.

Following this strategy can help you find the source of the misfire sooner.

A leaking EGR valve can lead to a random misfire.

A leaking EGR valve can lead to a random misfire.

An air/fuel ratio that goes outside its normal parameters and the computer is unable to adjust for will usually cause misfires and trigger diagnostic trouble codes, for example, when fuel is too lean (P0171, P0174) or too rich (P0172, P0175).

These codes don't necessarily mean there's a fault in the fuel system. But data, for example, from air/fuel or oxygen sensors can cause the PCM to make significant changes to ignition timing and pulse width of fuel injectors.

Problems that may cause a lean condition:

  • A bad oxygen sensor
  • Vacuum leak (possible symptoms: rough idle, stalling or fast idle)
  • Bad engine coolant temperature sensor
  • EGR valve leaking into the intake manifold
  • Faulty mass air flow (MAF) sensor
  • Faulty fuel pressure regulator
  • Dirty or clogged fuel filter
  • Dirty or clogged fuel injector
  • Bad throttle position (TP) sensor
  • Bad or worn fuel pump
  • Leaking intake manifold gasket

A leaking fuel injector or fuel pressure regulator can cause a rich condition.

Also, check for unmetered air past the MAF sensor. For example, a duct in the air cleaner assembly between the MAF sensor and throttle body may not be properly connected or may be damaged.

Other systems that can affect fuel trim

Problems in other systems may have an impact on fuel control operation as well. This includes:

  • Engine coolant temperature
  • Exhaust system
  • PCV system
  • Air induction system
  • Ignition system
  • Air/Fuel sensor and oxygen sensor

If trouble codes point to any sensor(s) in any of these systems, including the oxygen sensor, check those other system first since they may be affecting operation of the oxygen sensor(s).

Vehicles with a distributor, you can check and set the ignition timing.

Vehicles with a distributor, you can check and set the ignition timing.

3. Ignition Timing Misfires

Misfires can also be caused by ignition timing that is too advanced.

The car computer uses base timing for proper operation of those systems it controls.

It uses programmed data as well as input from sensors that monitor engine coolant temperature, engine speed, load, throttle position and others to make timing adjustments.

On vehicle models equipped with a distributor, you can check and adjust ignition timing.

On modern vehicles with electronic ignition (EI), you still can check ignition timing, but only a few models may allow for timing adjustment. Consult your vehicle repair manual for more information on checking and adjusting timing, if necessary.

Common Ignition Timing Issues

A worn or bad timing belt, chain, or tensioner will upset ignition timing.

Also, a common problem source on EI systems is the air gap for the crankshaft and camshaft position sensors. On some models, you can check and adjust the gap. So check your vehicle repair manual.

If the air gap is wrong, but you can't adjust it, you need to replace the sensor(s).

Other ignition primary circuit components you may want to check, depending on your model:

  • Ignition switch
  • Ignition coil
  • Pickup coil
  • knock sensor
  • Ignition control module

Symptoms of too-advanced ignition timing include pinging or engine knock.

When checking the ignition system, test the ignition coil, if necessary.

When checking the ignition system, test the ignition coil, if necessary.

4. Ignition System Misfires

The ignition system is a common source of misfires, specially when system maintenance has been neglected.

Worn or faulty spark plugs, wires, and ignition coils are common offenders. But even greasy spots left on newly installed spark plugs can lead to misfires.

Check your car manufacturer's service schedule. If you find ignition components in need of attention, check and replace them as necessary. This includes spark plugs, wires, coils, distributor cap and rotor.

When replacing spark plugs, verify the new plugs' gap using a wire feeler gauge. Your car owner's manual or repair manual will have the gap specification for your particular model.

What to check for:

  • Visually inspect spark plugs for fouling
  • Check spark plugs gap
  • Visually inspect and test spark plug wires
  • Check the ignition coil(s)
  • Distributor cap and rotor for carbon trace and damage

On systems with multiple coils, swap coils to see if the misfire follows the original coil. If so, the coil should be replaced.

Bad Grounds

Bad engine grounds also can lead to problems with the ignition system. Check for loose, corroded or disconnected ground straps. Sometimes, this may lead to burned ignition modules (bad coil ground straps), intermittent ignition faults (bad control module ground)

Also, check for engine ground voltage drops, if necessary.

Make sure ignition control modules are correctly mounted, and the surface is clean. Electrical connections at the module should be clean and tight.

For specific test you can conduct on secondary ignition system components for your vehicle, consult your vehicle repair manual.

Engine mechanical problems can lead to random misfires as well.

Engine mechanical problems can lead to random misfires as well.

Engine compression problems can also lead to random misfires.

  • Leaking valves and other valve train problems
  • Worn piston rings
  • Blown cylinder head gasket
  • Bad timed camshaft
  • Worn or scratched cylinder walls
  • Bad catalytic converter

Checking your engine with a vacuum gauge can give you valuable information about the internal state of your engine.

A vacuum gauge can reveal issues like:

  • worn rings
  • valve problems
  • worn cylinder walls
  • leaking intake manifold gasket
  • restricted exhaust
  • leaking head gasket
  • weak valve springs
  • stuck or burned valves
  • blocked catalytic converter

One or more of these components can contribute to the issue when going bad.

On some vehicles, you can find several failed components that can lead to random misfires, fix them, and still the misfire will persist, as you can see in the video below. On this case, a clogged catalytic converter was the issue.

Just keep in mind, if ignition timing is incorrect, it will affect your manifold vacuum readings.

Another valuable tool to learn about the mechanical condition of your engine is the compression gauge.

You can use a compression gauge in your own garage to diagnose problems with:

  • pistons
  • rings
  • valves
  • head gasket

You can conduct a compression test as a complementary diagnostic to a vacuum test. Consult your vehicle repair manual, if necessary.

Diagnosing with model specific scan tools will help with difficult to locate a random misfire source.

Diagnosing with model specific scan tools will help with difficult to locate a random misfire source.

6. Getting Professional Help

Random or intermittent misfires can not only be hard to diagnosed, but can lead to:

  • engine damage
  • damage to the catalytic converter and other systems, specially if the check engine light is flashing
  • unsafe driving conditions

So diagnosing the cause of a random misfire soon is important.

However, a diagnostic is made more difficult because each manufacturer may use different parameters, depending on the model, to trigger the check engine light due to a random misfire.

Thus, when you are having difficulty locating the source of a misfire, it's a good idea to have the car repair manual for your specific vehicle make and model.

Also, taking your vehicle to a shop for a diagnostic using a professional or model specific computer scan tool can make your repair project easier, even if you plan to make the repair yourself.

A professional scan tool can retrieve not only regular and pending trouble codes, but freeze-frame data, which cylinder has been the most affected, and other information that may help you locate the source of the problem quicker.

As a strategy, it's good practice to keep track of the time when a particular issue, especially P0300 random misfire, shows up. If you know, for example, that your vehicle began to act differently soon after a specific repair or engine service was performed, it might give you the clue to the source of the issue.

On some vehicles, you can find several failed components that can lead to random misfires, replace them, and the misfire may persist, as you can see in the video below.

So don't hesitate to invest in a manual or get professional diagnostic help when your own procedures come up empty. This also can save you time and money in the long run.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: My car has a P0300 error code. Can a bad ground cause a random misfire? There are two cylinders and I've tried everything that might cause this. I have problems with all my lights going bright and then dim.

Answer: It’s possible if a bad ground is affecting the charging system or the ECU. You might want to check the voltage drop for the charging circuit and engine grounds. This other post may help:

© 2019 Dan Ferrell