One of the most common signs of ignition system problems is engine misfiring. This occurs when one or more spark plugs fail to ignite the air-fuel mixture properly, often due to faulty ignition coils, worn-out spark plugs, or damaged spark plug wires. Misfires result in rough idling, hesitation during acceleration, and uneven engine performance (Delgado & Stockel, 2020). 

When one or more cylinders fail to fire correctly, the engine can become unbalanced, resulting in noticeable vibrations and a rough idle. This is often the result of a failed coil-on-plug unit, spark plug wear, or degraded spark plug wires (Delgado & Stockel, 2020). 

A weak or failing ignition system can make it hard for the engine to start. If the ignition coil or crankshaft position sensor is not working properly, the necessary spark may not be generated, preventing combustion. In some cases, the engine may crank but fail to start entirely (AutoZone, n.d.). 

Many modern vehicles will detect misfires or ignition coil problems and trigger the check engine light (CEL). A diagnostic scan (OBD-II) will often return codes like P0300–P0308 (cylinder misfire) or P0351–P0358 (coil circuit failure).

Modern vehicles are equipped with onboard diagnostics (OBD-II) systems that detect and log engine problems. If the ignition system is malfunctioning—due to misfires or coil circuit faults—the check engine light will likely illuminate. Diagnostic trouble codes such as P0300 (random misfire) or P0351–P0358 (ignition coil circuit faults) can help identify the exact issue (OBD-Codes.com, n.d.). 

If combustion is incomplete due to weak or inconsistent spark, more fuel is consumed to maintain power. This leads to lower gas mileage and increased emissions. Over time, it also stresses the catalytic converter.

When the ignition system fails to deliver a strong and timely spark, fuel may not burn completely. Incomplete combustion forces the engine to use more fuel to maintain power output, leading to increased fuel consumption and lower miles per gallon (Firestone Complete Auto Care, n.d.). 

Poor ignition can cause unburned fuel to enter the exhaust system, creating black smoke or a strong fuel smell. This indicates inefficient combustion and may lead to catalytic converter damage.

In engines with multiple ignition coils or coil-on-plug systems, a failing coil can cause one cylinder to misfire consistently. This misfire leads to excessive fuel delivery, poor combustion, and a sharp rise in emissions. The oxygen sensor and catalytic converter may also be affected over time, reducing their ability to clean the exhaust gases efficiently.

Additionally, worn or fouled spark plugs can cause delayed ignition or weak sparks, resulting in a fuel-rich mixture that does not burn efficiently. The ECU may try to compensate by adjusting the fuel-air ratio, which often worsens the problem.

Intermittent stalling, especially while idling or at low speeds, can signal an ignition issue. If the ECU loses track of timing due to sensor or coil failure, it may shut the engine down unexpectedly — a safety risk if it happens while driving.

Causes of Stalling from Ignition Failures:

1. Faulty Ignition Coil: A weak or failed ignition coil cannot generate the high-voltage spark needed for combustion. This may result in misfires or engine shutdown, especially under load or during acceleration (Firestone Complete Auto Care, n.d.).
   
   

2. Failing Crankshaft or Camshaft Position Sensors: These sensors provide the ECU with crucial data to time the spark accurately. If they malfunction, the ECU may not know when to fire the spark, causing the engine to stall or fail to restart (Monolithic Power Systems, 2020).
   
   

3. Bad Spark Plugs or Wires: Worn, fouled, or damaged spark plugs and ignition wires may produce inconsistent or weak sparks, especially under changing engine conditions, leading to rough idling and stalling.
   
   

4. Ignition Switch Issues: A failing ignition switch can intermittently cut power to the ignition circuit, shutting off the engine unexpectedly even while driving.
   
   

5. Intermittent Electrical Connections: Loose or corroded electrical connectors related to the ignition system can cause temporary power loss, resulting in sporadic stalling that is hard to predict or replicate.

Older systems with distributors or cracked spark plug wires can misfire more easily in rainy or humid weather, due to water affecting the spark path.

How Moisture Affects Ignition Systems:

1. Distributor Caps: In older ignition systems, moisture inside the distributor cap can cause arcing between terminals or prevent the rotor from directing current correctly to the spark plugs. This can result in misfires, hard starting, or stalling.
   
   

2. Spark Plug Wires: Water can cause current to leak through worn or cracked wires, leading to a weak or erratic spark. This leakage often goes unnoticed until the engine is exposed to wet conditions, when performance suddenly drops.
   
   

3. Ignition Coils: In systems like COP (Coil-On-Plug), moisture can enter through the coil boots and cause corrosion or short circuits. While more sealed than traditional systems, they are not entirely immune to environmental exposure.
   
   

4. Electrical Connections: Moisture can enter corroded or unsealed connectors, especially around crankshaft sensors or ignition modules, creating intermittent failures that are difficult to trace.

A loud backfire from the tailpipe often means that combustion is happening late — sometimes outside of the cylinder. This can be caused by ignition timing issues, faulty plugs, or failing coils.

How the Ignition System Contributes to Backfiring:

1. Incorrect Ignition Timing: A worn or failing ignition coil, crankshaft sensor, or misprogrammed ECU can delay spark timing. This causes the air-fuel mixture to ignite after the exhaust valve opens, resulting in a sharp pop in the exhaust (AutoZone, n.d.).
   
   

2. Weak Spark or Misfire: If the spark plug does not ignite the fuel properly due to coil or plug failure, the unburned fuel is pushed into the exhaust system, where it can ignite due to residual heat or a delayed spark.
   
   

3. Distributor Issues (in Older Systems): In vehicles with a mechanical distributor, wear or moisture in the distributor cap or rotor can result in spark being delivered to the wrong cylinder or at the wrong time, causing combustion to occur at incorrect intervals (Delgado & Stockel, 2020).
   
   

4. Faulty Spark Plug Wires: Cracked or deteriorated spark plug wires can cause cross-firing (where spark jumps to the wrong cylinder), leading to premature or delayed ignition and backfiring.

Cracked ignition coils, worn spark plug wires, or carbon-tracked distributor caps (on older systems) are often visible signs of a failing ignition system. Inspection under the hood may reveal these clues before performance issues become serious.

Common Visual Signs of Damage:

1. Ignition Coil Cracks or Burn Marks
   Ignition coils convert low battery voltage into the high voltage needed to spark the air-fuel mixture. If a coil shows visible cracks or burn marks, especially around the casing or electrical terminals, it can cause weak or no spark, leading to misfires and engine hesitation (Delgado & Stockel, 2020).
   
   

2. Corroded or Melted Connectors and Terminals
   The electrical connections on ignition coils, spark plug wires, and modules can become corroded due to moisture or heat. Melted plastic, discolored terminals, or green/white corrosion buildup can disrupt voltage flow and cause intermittent failures (AutoZone, n.d.).
   
   

3. Damaged Spark Plug Wires
   Cracked, brittle, or burned spark plug wires can cause arcing—where electricity jumps to nearby metal parts instead of reaching the plug. This not only reduces engine efficiency but also increases the risk of backfiring or stalling (Firestone Complete Auto Care, n.d.).
   
   

4. Worn or Cracked Distributor Cap and Rotor (for older vehicles)
   In mechanical ignition systems, the distributor cap and rotor direct the spark to the correct cylinder. Cracks in the cap or corrosion on the contacts can lead to cross-firing, delayed ignition, or complete failure to spark.