How do you diagnose what is causing a misfire?

Start with an OBD-II scan to identify misfire codes (P0300 for random/multiple misfires or P0301–P0308 for cylinder-specific misfires), determine whether the misfire is random or tied to a specific cylinder, and then follow a structured test of ignition, fuel delivery, air intake, and engine timing to pinpoint the fault.

Misfires disrupt power, fuel economy, and emissions. They can stem from worn spark plugs, faulty ignition coils, clogged injectors, vacuum leaks, sensor glitches, or timing/ compression problems. This guide outlines a systematic diagnostic approach, combining vehicle data with practical checks that work for DIYers and professionals alike.

Understanding how misfires are detected and categorized

Engine misfires are diagnosed primarily through trouble codes and real-time data. Knowing whether the code is for a single cylinder or a random misfire helps target the checks you perform next. This section outlines how to interpret the most common codes and data patterns you’ll encounter.

1. Pull and review OBD-II codes: P0300 indicates random/multiple misfires; P0301–P0308 point to a specific cylinder. Note any pending codes and how long the condition has been present.


2. Check freeze-frame data: review engine RPM, load, coolant temperature, fuel trims, and other sensor readings at the moment the misfire occurred to narrow down likely causes.


3. Look at live data and misfire counters: observe cylinder-specific misfire counts, ignition advance, air/fuel ratios, and fuel trim trends to distinguish electrical from fuel or air issues.


4. Consider vehicle-specific diagnostics: some modern cars show additional fault codes for sensors, timing, or communication issues that can accompany a misfire.

Concluding: A clear pattern in codes and data helps you triage ignition, fuel, air, or timing problems before you start exchanging parts.

Ignition system checks

Ignition problems are a frequent source of misfires. This section covers the components to inspect and the tests to perform to confirm ignition-related faults.

1. Spark plugs: inspect for wear, fouling, gapping, and heat-range suitability; replace if worn or contaminated.


2. Ignition coils or coil packs: check for proper resistance and function; swapped or damaged coils can produce misfires on specific cylinders.


3. Ignition wiring and connectors: inspect for damaged insulation, loose connections, and corrosion that can cause intermittent sparks.


4. Spark quality testing: use a spark tester or oscilloscope to verify a strong, consistent spark waveform across affected cylinders.


5. When applicable, verify coil-to-plug boot seating and any ignition control module signals from the ECM/PCM.

Concluding: Replacing worn plugs and defective coils or repairing faulty wiring often resolves ignition-origin misfires, though some engines require recalibration or software updates to address control logic.

Fuel system checks

Fuel delivery and injector performance are common misfire sources, especially if there are pressure or flow irregularities or abnormal fuel trims.

1. Fuel pressure/volume: compare measured pressure to spec and test for adequate volume; a weak pump or restricted return line can cause lean or rich misfires.


2. Injector operation: ensure injectors are opening/closing as commanded; balance or spray-pattern tests can reveal sticking or leaking injectors.


3. Fuel trims: monitor short- and long-term fuel trims (STFT/LTFT); persistent rich/lean trims indicate a fueling or sensor problem.


4. Fuel quality and contamination: check for contaminated fuel, water in the tank, or a failing fuel filter that restricts flow.

Concluding: Ignition may be fine while a fuel delivery issue causes a misfire; addressing fuel pressure, injector performance, and abnormal trims can restore smooth running.

Air intake, vacuum leaks, and sensors

Air delivery and intake-side sensors play a critical role in engine combustion. Leaks or faulty sensors can mimic or cause misfires by altering the air–fuel mixture or timing signals.

1. Vacuum and intake leaks: perform a smoke test or spray-down test around hoses, gaskets, and the intake manifold to locate leaks that lean out the mixture.


2. Mass airflow (MAF) and manifold absolute pressure (MAP) sensors: verify readings are within spec and that wiring is intact; contamination or incorrect readings can skew air measurement.


3. Other sensors: crank/cam position sensors, throttle position sensor, and O2 sensors can influence ignition timing and fuel trims; diagnose sensor faults with live data and spec checks.


4. PCV valve and vacuum routing: ensure the PCV system is functioning properly and that no unwanted leaks or blockages exist.

Concluding: Fixing leaks, cleaning or replacing sensors as needed, and ensuring clean, accurate air measurement are essential steps to correcting misfires caused by air and sensor faults.

Mechanical timing and compression checks

If ignition and fueling appear sound, the problem may lie in timing or compression. This subsection covers the mechanical checks that identify timing or internal engine issues.

1. Timing integrity: verify that the timing belt/chain and tensioners are correct and that timing marks align as specified; a slipped timing belt is a common misfire trigger.


2. Compression testing: perform dry compression checks across all cylinders; a low reading in one or more cylinders indicates mechanical wear or seal issues.


3. Wet vs dry compression: add a small amount of oil to a suspect cylinder and re-test; a rise in compression suggests worn rings or valve sealing problems.


4. Leak-down testing: pinpoint the source of compression loss (valves, rings, or head gasket) by pressurizing each cylinder and listening for leaks.

Concluding: When timing or mechanical integrity is at fault, replacing timing components or addressing engine wear often resolves persistent misfires that ignition and fuel checks could not fix.

Verification, road test, and follow-up

After repairs, validate the fix with a controlled test and monitor for recurrence. A thorough road test ensures the misfire is resolved under real driving conditions.

1. Clear codes and perform a road test: drive under typical conditions to verify that the misfire does not reoccur and to observe real-world performance.


2. Recheck data: scan for new or cleared codes and monitor live data for stability in ignition, fuel trims, and sensor readings.


3. Reassess fuel economy and emissions: if the vehicle previously failed emissions tests, re-check that the catalyst and O2 sensors respond normally after the repair.

Concluding: A successful repair should result in no active misfire codes, stable sensor readings, and a clean test drive without hesitation, roughness, or drop in RPM.

Summary

Diagnosing a misfire requires a structured approach: start with an OBD-II data review to categorize the misfire, then methodically test ignition, fuel delivery, air intake, sensors, and timing. Use exchange tests (swap components), controlled measurements (compression, fuel pressure, ignition waveform), and road testing to confirm the root cause. Whether you’re a DIYer or a professional, a disciplined diagnostic workflow minimizes guesswork, reduces unnecessary parts replacements, and restores engine performance and efficiency.

Why is my engine still misfiring after replacing spark plugs and coils?

Your car may still be misfiring after changing the spark plugs and ignition coils because the problem lies with other components such as the fuel injectors, vacuum leaks, worn spark plug wires, or low engine compression. Other potential causes include issues with the fuel delivery system, airflow to the engine, or ignition timing.
 
This video explains the possible reasons why your car is still misfiring after changing the spark plugs and ignition coils: 59sFixITYouTube · Oct 3, 2024
Other potential causes 

• Fuel system issues: A dirty or malfunctioning fuel injector, a clogged fuel filter, or low fuel pressure can lead to improper fuel delivery. 
• Vacuum leaks: Leaks in the intake manifold or other vacuum lines can cause an imbalance in the air-fuel mixture. 
• Spark plug wires: Even with new coils, old or damaged wires can prevent a strong spark from reaching the plugs. 
• Low compression: This can be caused by issues like a worn piston ring, a leaky valve, or a blown head gasket, all of which prevent the cylinder from compressing the air-fuel mixture effectively. 
• Incorrect timing: If the ignition timing is off, the spark plug won't fire at the right moment for efficient combustion. 
• Faulty new parts: It's possible that one of the "new" parts is defective, so it's worth swapping the new coil with one from a cylinder that isn't misfiring to see if the problem follows the coil. 
• Wiring and connector issues: Corroded, damaged, or loose connectors to the coils or fuel injectors can cause misfires. 
• Carbon buildup: Excessive carbon deposits on the intake valves can prevent them from sealing properly, especially in direct-injection engines. 

How to tell which cylinder is misfiring without a scan tool?

To find a misfiring cylinder without a scan tool, perform a "cylinder balance test" by carefully and one-by-one disconnecting the ignition coil on a running engine; the cylinder that shows no change in engine RPM when its coil is removed is likely the one misfiring. Alternatively, use an infrared thermometer to check exhaust manifold temperatures, as a misfiring cylinder will have a noticeably colder port.
 
This video demonstrates how to perform a cylinder balance test: 59strackpalmerYouTube · Feb 2, 2024
Cylinder balance test

• Procedure: With the engine running, carefully disconnect the ignition coil (or spark plug wire) from one cylinder at a time. 
• What to listen for:
  • Normal cylinder: When you disconnect the coil, the engine's RPMs will drop and the engine will run even rougher. 
  • Misfiring cylinder: If there is no noticeable change in the engine's running or sound when you disconnect the coil, that cylinder is the one that is misfiring. 
• Safety note: Be careful to avoid electrical shock by not touching any live electrical components.

Infrared thermometer test 

• Procedure: After the engine has been running for a short period, use an infrared thermometer to measure the temperature of each exhaust manifold port where it exits the cylinder head.
• What to look for:
  • Properly firing cylinders will have similar, high temperatures, often around 200∘F200 raised to the composed with power cap F200∘𝐹 or hotter.
  • The misfiring cylinder's exhaust port will be significantly cooler than the others. 

You can watch this video to learn how to use an infrared thermometer to diagnose a misfire: 57s2CarProsYouTube · Jun 28, 2017
Next steps

• Once the misfiring cylinder is identified, you can further test its components. 
• Swap the suspected bad coil with one from a known-good cylinder. If the misfire moves to the other cylinder, the coil is the problem. 
• If the misfire does not follow the coil, the issue could be the spark plug or a problem with the fuel injector or compression. 
• You can then remove the spark plug from the misfiring cylinder to inspect its condition (for example, look for fouling). 

How to figure out what's causing a misfire?

This short guide can assist you in troubleshooting a misfire, helping you to diagnose the issue in order that it can be repaired quickly.

1. Acceleration or idle is rough.
2. Check Engine Light is on.
3. Bad ignition coil on an older vehicle.
4. Worn spark plugs.
5. Gasket leaks.

What are the three most common causes of a misfire?

Three common causes of an engine misfire are problems with the ignition system, such as worn-out spark plugs or faulty ignition coils; issues with the fuel system, like clogged fuel injectors or a weak fuel pump; and air-related problems, such as vacuum leaks.
 
Ignition system issues

• Spark plugs: Worn-out, dirty, or improperly installed spark plugs are a very common cause of misfires. 
• Ignition coils: A bad or malfunctioning ignition coil can fail to provide the proper spark to the spark plug. 
• Spark plug wires: Damaged or worn-out spark plug wires can also prevent the spark from reaching the cylinder. 

Fuel system problems

• Fuel injectors: Injectors can become clogged with deposits, restricting the flow of fuel. 
• Fuel pump: A weak fuel pump may not provide enough pressure for proper combustion. 
• Fuel filter: A dirty or clogged fuel filter can restrict the flow of fuel to the engine. 

Air-related problems

• Vacuum leaks: Leaks in the vacuum system allow extra, unmetered air into the engine, disrupting the correct air-fuel mixture. 
• Mass airflow (MAF) sensor: A faulty MAF sensor can send incorrect information about the amount of air entering the engine.