Encountering a Check Engine Light can be unsettling for any car owner. Recently, my wife experienced just that with our trusty, high-mileage Volvo S60. After a blood donation trip, she reported a “rough” running engine accompanied by the dreaded glow of the Check Engine Light. With 280,000 miles on the odometer of this naturally aspirated S60, and having previously addressed ignition coil issues on cylinders 2 and 5, I knew it was time to investigate. Aside from routine maintenance like timing belt replacements and throttle body cleanings, the engine’s internals have remained untouched, making any new issue a point of focused troubleshooting.
My initial step was to drive the car myself to better understand the symptoms and, crucially, to retrieve the diagnostic trouble codes (DTCs). Using an OBDII scanner, the code P0303 surfaced, pinpointing a misfire in Cylinder 3. Having seen similar codes before, which sometimes resolved themselves, I was aware that misfires can be transient. However, this particular P0303 code had persisted for approximately 40 miles, signaling a more persistent problem.
Based on my experience, a P0303 code typically boils down to three potential culprits:
- Faulty Spark Plug: A worn, fouled, or damaged spark plug can fail to ignite the air-fuel mixture effectively.
- Defective Ignition Coil: The ignition coil is responsible for delivering the high-voltage spark to the spark plug. A failing coil can lead to insufficient or no spark.
- Problematic Fuel Injector: A clogged or malfunctioning fuel injector can disrupt the proper fuel delivery to the cylinder, causing a lean mixture and misfire. A leaking injector seal could also contribute to fuel delivery issues.
Step 1: Spark Plug Inspection and Replacement
Starting with the most accessible and common cause, I decided to inspect and replace the spark plugs. The Volvo S60 utilizes multi-electrode Bosch spark plugs, factory-gapped and non-adjustable. These plugs had been in service for just over 80,000 miles, nearing their expected lifespan.
Upon removal, all spark plugs exhibited significant deposits. However, spark plug #3, corresponding to Cylinder 3, stood out. It displayed black soot on its firing tip and a tan ring around the porcelain insulator above the hex head, indicative of potential overheating. This visual inspection alone suggested an issue within Cylinder 3.
Step 2: Examining the Ignition Coils
Next, I turned my attention to the ignition coils. Removing the coil packs, I visually compared them. While most appeared uniform, the ignition coil for Cylinder 3 presented a “shiny” appearance. Furthermore, a trace of oil was noticeable on the rubber gasket that seals the coil to the spark plug well.
Cylinder 3 is positioned centrally, adjacent to the oil fill cap. Initially, I dismissed the oil as potential spillage from oil refills. However, this observation would become more significant later.
After replacing the spark plugs with new ones, I test-drove the car. While the severity seemed slightly reduced, a subtle misfire was still perceptible, particularly at idle. The Check Engine Light remained, and the P0303 code reappeared upon rescanning. This indicated that the spark plugs alone were not the sole solution.
Step 3: Ignition Coil Swap and the Oil Leak Revelation
Moving on to the ignition coil, I decided to replace the Cylinder 3 coil with a spare I had available. Upon removing the original coil and setting it aside horizontally, a crucial discovery was made: oil was actively dripping from the coil head. Further inspection, by pulling back the boot, revealed the interior was saturated with oil.
Despite the oil presence within the coil itself, the spark plug well remained surprisingly clean and dry, exhibiting the typical bright silver aluminum finish. This observation clarified that the oil was not external contamination but rather a leak originating from within the ignition coil itself.
The oil within an ignition coil serves a vital purpose: insulation and cooling of the high-voltage components. When this oil leaks, it compromises the coil’s insulation, increasing the risk of electrical arcing and misfires. Unlike previous ignition coil failures I had encountered, which involved carbon tracking within the boots, this instance presented a clear oil leak as the failure mechanism.
Following the ignition coil replacement, I started the engine. Immediately, the engine ran smoothly. I cleared the stored OBDII codes and took the car for an extended test drive. The misfire was completely resolved, and the Check Engine Light remained off.
Lingering Concerns and Preventative Measures
While the P0303 misfire was successfully addressed by replacing the ignition coil, a lingering concern remains about potential long-term effects. A persistent misfire can lead to unburnt fuel entering the combustion chamber. This “cylinder wash” can strip away the protective oil film on the cylinder walls, potentially leading to increased wear and even scoring of the cylinder bore by the piston rings.
To assess any potential damage, a compression test could be beneficial to compare the compression readings of Cylinder 3 with the other cylinders. However, the accessibility of the spark plug cavity for a compression gauge needs to be considered.
Although a fuel injector issue was considered a possible cause, the successful resolution with the ignition coil replacement suggests it was not the primary culprit in this case. Nevertheless, having a spare fuel injector on hand for future contingencies is a prudent measure.
This experience underscores the importance of systematic troubleshooting when addressing OBDII codes like P0303. Starting with basic checks like spark plugs and ignition coils, and progressing methodically, can effectively diagnose and resolve engine misfires. In this instance, a leaking ignition coil was identified as the root cause, highlighting a less common but critical failure mode. Regular maintenance and prompt attention to engine performance changes are crucial for preventing minor issues from escalating into more significant problems.
