Customer concern
More than a routine service
This Renault Clio was booked in with MPH Northwest for a routine full service. Alongside the scheduled maintenance, the customer reported that the engine had been experiencing a misfire during cold running.
The problem was most noticeable when the engine was cold and improved as the vehicle warmed up. Rather than treating the symptom separately from the service, it was investigated as part of the vehicle's overall maintenance assessment.
Inspection & diagnosis
Investigating the ignition system
As part of the service and investigation into the reported misfire, the ignition system was inspected. With the vehicle approaching 40,000 miles, replacement of the spark plugs was recommended as part of the maintenance work.
Once removed, a significant difference was identified between the three spark plugs. The plug from Cylinder 2 showed excessive wear to its fine iridium centre electrode compared with the plugs removed from Cylinders 1 and 3.
Technical finding
An excessively enlarged spark gap
The deterioration of the Cylinder 2 centre electrode had resulted in an excessively enlarged spark gap. This meant the ignition system had to generate a higher voltage for the spark to bridge the electrodes.
The abnormal wear provided a strong explanation for the intermittent misfire the customer had experienced during cold running.
Physical inspection still matters
Comparing the removed components revealed a clear difference between cylinders. Cylinder 2 had deteriorated significantly more than the remaining spark plugs, providing useful evidence when investigating the customer's complaint.
Repair
Correct-specification spark plugs installed
A complete set of the correct-specification replacement spark plugs was installed as part of the service.
With the maintenance work complete, the engine's operation was checked to assess the result of the repair and confirm that the reported symptom had been resolved.
Final result
Cold-running misfire resolved
Following replacement of the spark plugs, the cold-running misfire was no longer present and normal engine operation was restored.
Although iridium spark plugs are designed for an extended service life, this case demonstrates why vehicle symptoms and physical inspection remain important alongside scheduled maintenance intervals. Components can deteriorate unevenly, and identifying developing problems early can help prevent poor running from progressing into more significant faults.