How to Fix Engine Misfire – Identify and Diagnose

Engine misfire can significantly impact performance and efficiency and caused by spark plugs, timing, fuel system, or an ignition coil. Let’s walkthrough what is happening and how to correct it.

What is an Engine Misfire?

An engine misfire occurs when one or more cylinders fail to ignite properly. This can lead to uneven power delivery, affecting your car’s overall performance. Symptoms include rough idling, power loss, and increased emissions.

Common Causes of Engine Misfire

Spark plug issues are a frequent cause of engine misfires. A faulty ignition coil can also disrupt the engine’s normal firing sequence. Clogged fuel injectors impede proper fuel delivery, leading to misfires. Low fuel pressure or dirty filters can disrupt the fuel supply. Timing belt or chain problems affect the engine’s synchronization, causing misfires.

Diagnosing Engine Misfire

A diagnostic scan tool can identify which cylinder is misfiring. Visual inspection of spark plugs can reveal wear or damage. Testing an ignition coil with a multimeter helps find electrical faults. Fuel pressure gauges assess fuel pump and injector performance. Checking timing belts or chains for wear is crucial for engine timing.

Spark Plugs – Misfire

1. Locate Spark Plugs:
   • Find the spark plugs in your engine; they’re usually attached to thick wires.
   • Your vehicle’s manual can help you identify their exact location.
2. Assess Spark Plugs
   • Remove the plug wire by gently twisting and pulling it off.
   • Use a spark plug socket and ratchet to remove the spark plug.
   • Examine the spark plug for wear, fouling, or damage.
3. Replace Spark Plugs (if necessary):
   • If damaged or worn out, buy the correct replacement plugs for your vehicle.
   • Screw in the new spark plug by hand, then tighten with the socket and ratchet.
   • Reattach the plug wire securely.

Ignition Coil

• • Ensure the engine is off and cool.
  • Disconnect the battery to avoid any electrical hazards.
  • Refer to your vehicle’s manual to find the ignition coils. They’re typically in or near the engine, connected to the spark plugs.
  • Set your multimeter to the ohms (Ω) setting to measure resistance.

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Test Primary Ignition Coil Resistance:

• • Locate the positive (usually marked with a “+”) and negative terminals on the coil.
  • Place the multimeter leads on these terminals.
  • The typical reading for primary resistance should be between 0.4 and 2 ohms, but check your manual for the specific range for your vehicle.

Test Secondary Ignition Coil Resistance:

• • Locate the high tension terminal (where the spark plug connects) and the negative terminal.
  • Place one lead of the multimeter on the high tension terminal and the other on the negative terminal.
  • This reading is usually much higher, typically between 6,000 to 15,000 ohms, but again, refer to your vehicle’s specifications.

Interpreting Results:

• • Readings fall outside the specified range for your vehicle, the coil is likely defective and needs replacement.
  • No continuity (the multimeter shows an infinite resistance), it’s a sign of an open coil and needs replacement.
  • Resistance is much lower than the specified range, it indicates a shorted coil, also requiring replacement.

Additional Tips

• • Test all coils in your vehicle for consistency.
  • Some modern vehicles may have more complex coil setups or integrated coil packs. Refer to the manual for specific procedures related to your model.

Fuel Injectors

• • Consult your vehicle’s manual to find the injectors. They are usually on or near the engine, attached to the fuel rail.

Visually Inspect the Injectors:

• • Check for any visible signs of damage or leaks.
  • Ensure the electrical connectors are secure and in good condition.

Test Injector Clicking (Functional Test):

• • Start the engine or turn the ignition to the “on” position.
  • Use a long screwdriver or an automotive stethoscope to listen to each injector.
  • Place the tool tip on the body of the injector and your ear on the other end.
  • A healthy injector will make a regular clicking sound. No sound or an irregular pattern may indicate a problem.

Test Injector Resistance:

• • Set the multimeter to measure resistance (ohms).
  • Disconnect the electrical connector from the injector.
  • Place the multimeter leads on the injector terminals.
  • Measure the resistance and note the value.
  • Compare the reading to the specifications in your vehicle’s service manual. Typical values range from 12 to 16 ohms, but this can vary.

Interpreting Results:

• • If the resistance is significantly higher or lower than specified, it indicates a problem with the injector.
  • Inconsistent readings between injectors can also signify issues.

Check Wiring and Connectors:

• • Inspect the wiring and connectors for damage or corrosion.
  • Issues with the electrical connections can also cause injector problems.

Reconnect the Injectors:

• • After testing, make sure to reconnect all electrical connectors securely.

Additional Tips

• While the resistance test can indicate electrical issues, it doesn’t test the injector’s ability to spray fuel correctly.
• A professional-grade scan tool can perform more advanced diagnostics, like pulse width and response time tests.
• Injectors that fail the tests should be cleaned or replaced. Sometimes, poor performance is due to clogging rather than electrical failure.
• Do not attempt advanced diagnostics if you are not familiar with automotive electrical systems.

Fuel System

In addition to replacing fuel filters, the entire fuel system can be tested.

Depressurize the Fuel System:

• • Relieve the fuel system pressure to prevent fuel spray when disconnecting the fuel line. This can typically be done by removing the fuel pump fuse or relay and running the engine until it stalls.

Locate the Fuel Test Port:

• • Find the Schrader valve (similar to a tire valve) on the fuel rail in the engine compartment. If your vehicle doesn’t have a test port, you may need a special adapter to tap into the fuel line.
  • Connect the fuel pressure gauge to the test port. If your vehicle lacks a test port, install the appropriate adapter between the fuel line and the fuel rail.

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Start the Engine:

• • With the gauge securely attached, start the engine or turn the key to the “on” position to activate the fuel pump.

Read the Fuel Pressure:

• • Observe the reading on the fuel pressure gauge. Compare this reading to the specifications in your vehicle’s service manual.
  • The pressure should be within the range specified for your vehicle. It’s normal for the pressure to fluctuate slightly but it should generally be stable.

Check Pressure with Engine Off:

• • Turn off the engine and observe the pressure gauge. The pressure should not drop quickly; a rapid pressure drop can indicate a leak or a faulty fuel pressure regulator.

Inspect for Leaks:

• • Check the fuel lines, injectors, and connections for any signs of fuel leaks.

Check Fuel Pump Operation:

• • Listen for the fuel pump; it should run for a few seconds when you turn the key to the “on” position.

Safety

• • Avoid smoking or open flames when working on the fuel system.
  • If you detect any fuel leaks, address them immediately to prevent fire hazards.

Timing Belts or Chains

• • These are usually at the front of the engine, behind a cover.
  • Refer to the vehicle’s manual for specific location and procedure.

Assess Timing Belts/Chains:

• • Check for cracks, wear, or loosening in the belt or chain.
  • Inspect the tension and alignment.

Replace Timing Belts/Chains (if necessary):

• • This is a complex task, often requiring special tools and alignment.
  • It’s recommended to have this done by a professional mechanic.