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Diagnosing Engine Misfires: Tips and Strategies

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

With so many systems, it's hard to find the source of a misfire.

With so many systems, it's hard to find the source of a misfire.

Engine misfires can catch you off guard. You are driving smoothly down the highway; next thing you know the engine starts to buck and shake.

An engine cylinder misfires when it is unable to efficiently burn the air/fuel mixture in the combustion chamber. This problem leads to all kinds of engine performance issues: The engine jerks and loses power, and fuel consumption and emissions go up.

How Easy Is It to Diagnose a Misfire?

That depends on the root cause of the misfire. A misfire may originate in one or more systems where components have failed or aged. Sometimes misfires are hard to diagnose even for experienced technicians, especially intermittent misfires.

The good news is that most misfires happen because of lack of proper maintenance of common and accessible parts. So you have a chance of diagnosing the problem at home, and even fixing it, if you know how to go about it.

The first thing you want to know, though, is that you should start diagnosing the misfire on your car as soon as possible. Not only because you are wasting fuel and contaminating the environment unnecessarily, but also because your repair costs can go up significantly as time goes by without a repair. For one, your catalytic converter may fail due to unburned fuel.

This guide will help you find those common misfire sources. Before, though, let's take a look at the problem in a little more detail so that you know what kind of information to look for during your diagnostic procedure.

Index

I. What Can Cause a Misfire?

Symptoms to Watch Out for If You Suspect a Misfire

I. How to Save Diagnostic Time with a Scan Tool

II. Things to Do Before Starting an Engine Misfire Diagnosis

Misfire Related DTCs

III. Checking for Lack of Ignition Spark

A Trick To Diagnosing Misfires Quicker

IV. Checking Fuel Supply Problems

CEL or MIL Flashing - A Warning Sign

V. Checking for Vacuum Leaks

VI. What If My Ignition, Fuel and Vacuum Systems Turn Out OK?

I. What Can Cause a Misfire?

As mentioned above, a misfire is the failure of one or more cylinders to fire properly or at all. Misfiring means the combustion process—the igniting of the air-fuel mixture that enters the cylinder—has been upset in some way.

A misfire usually happens because of:

  • a component failure in the ignition system (including abnormal ignition timing advance),
  • problems in the fuel system,
  • vacuum leaks,
  • compression drop in one or more cylinders, or
  • problems with key valves or sensors the car computer uses to calculate the correct air-fuel ratio.

Misfires due to the absence of spark—as opposed to the absence of fuel—are particularly worrisome because the unburned fuel may find its way into the exhaust system and catalytic converter.

If this happens, the raw fuel will gradually destroy the converter; hence the need to fix the problem as soon as possible. On most modern vehicles, the Check Engine Light (CEL) or Malfunction Indicator Light (MIL) will begin to flash if a misfire threatens to damage your catalytic converter.

Symptoms to Watch Out for If You Suspect a Misfire

When an engine misfires, you may notice one or more of these symptoms:

* Engine loses power

* Engine is hard to start

* Fuel consumption increases

* Emissions increase

* Engine produces popping sounds

* Intake or exhaust manifold backfires

* Engine vibrates, jerks or stumbles

* Engine stalls

I. How to Save Diagnostic Time With a Scan Tool

Modern vehicles use highly accurate crankshaft position sensors that can detect crankshaft angle position at any speed. This greatly helps the Powertrain Control Module (PCM--car computer) calculate crankshaft acceleration time. The PCM can detect when a cylinder decelerates (an indication of a misfire), store a Diagnostic Trouble Code (DTC) in memory, and turn on the Check Engine Light. Some car computers detect misfires by measuring electrical activity [ionization] at the spark plug electrodes.

You can retrieve stored Diagnostic Trouble Codes with a code reader (for OBD I systems on pre-1996 vehicle models) or a generic scan tool (for OBD II systems-1996 or newer models; some scan tools can read both OBD I and II).

Depending on your vehicle make and model—and the sophistication of your scan tool—you may get this and other data from your car computer when diagnosing a misfire:

  • The cylinder or cylinders where the misfire or misfires were detected.
  • Whether the affected cylinder has actually fired since the problem was detected.
  • Number of misfires within a recent number of cycles.
  • Engine RPM when the misfire was detected.
  • Misfire history.

All these are important clues that car technicians used to locate the source of misfires.

If you don't own a scan tool, though, you can buy a relatively inexpensive, aftermarket scanner you can use for this diagnostic. A scan tool is a good investment because you can use it for other small repairs and maintenance tasks in the future.

Alternatively, if you don't want to buy a scan tool now, remember that some auto parts stores in your area may retrieve any stored computer codes in your vehicle without charge.

Not all misfires are directly translated into DTCs, especially intermittent ones. Yet, it's a good idea to scan your computer memory because other potential stored codes may help you in diagnosing and fixing your engine misfires (see the box below on Misfire-Related DTCs).

Whether or not you have access to a scan tool, this guide gives you important tips you can use to troubleshoot and fix the misfire in your vehicle.

Take a flashing Check Engine Light as a warning sign.

Take a flashing Check Engine Light as a warning sign.

II. Things to Do Before Starting an Engine Misfire Diagnosis

Before you start diagnosing the problem, try to gather as much information about the operating conditions in which the misfire occurs. The information that you gather gives you important evidence to help you pinpoint the cause behind the misfire. Think about the conditions in which the engine misfire occurs, for example:

  • Only when the engine is cold
  • Only when the engine warms up
  • Only when accelerating
  • It happens consistently
  • Only when it rains
  • Engine misfires randomly
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Read More from AxleAddict

Perhaps the misfire began after you, or someone else made a repair or performed some maintenance work (changed spark plugs). Maybe you or someone else left a hose loose or electrical connector loose or unplugged.

The next three sections—Ignition, Fuel, Vacuum—present the systems most likely to be behind the misfire. The section following those gives you a list and a brief explanation of other specific components that can also give you trouble. It is meant to help you dig a bit deeper when necessary.

The Powertrain Control Module (PCM-car computer) can be a valuable source of information for your diagnostic process. If you find one or more of the following Diagnostic Trouble Codes (DTCs), pair it with the information presented in this guide. Even if the code doesn't pinpoint the reason for the misfire, it can hint at the root of the problem.

Here are some common DTCs you may find in your car computer that can help diagnose a misfire:

  • P0100-P0104: Mass air flow sensor related codes.
  • P0171 or P0172: Lean or rich fuel conditions.
  • P0200 code series: Fuel injector circuit malfunctions.
  • P0220-P0229: Throttle or pedal position sensor related codes.
  • P0300: Multiple, random misfires.
  • P0301-P0312: The last two digits point to the cylinder number where a misfire happened.
  • P0335-P0339 - Crankshaft position sensor related codes
  • P0400: Some codes in this series indicate an EGR system malfunction.

If you have a pre-1996 vehicle, check the manual that comes with your code reader to see the definition of the codes you retrieve from your car computer.

Carefully check the condition of your spark plugs.

Carefully check the condition of your spark plugs.

III. Checking for Lack of Ignition Spark

Worn out or bad ignition system components are a common source of misfires. If you haven't inspected or replaced ignition components during the past 3 to 5 years, a worn or failed component in the system may be the cause.

If you retrieve a DTC pointing to a particular cylinder, concentrate on those components related to that cylinder. For example: