Mar 6 (edited) • General discussion
Case Study- Intermittent Misfire - 2012 Dodge Charger SRT8
Advanced Diagnostic Case Study: Intermittent Misfire With No Clear Cause
Here’s an interesting diagnostic situation that highlights why pattern recognition matters.
Vehicle came in with a complaint of:
• Roughness during light throttle cruise
• No noticeable misfire at idle
• Engine ran smooth during heavy acceleration
• Occasional P0300 random misfire code
Initial checks looked normal.
Ignition components were good:
- spark plugs inspected
- coils tested
- injector balance looked normal
Fuel pressure was within specification.
At first glance this looked like a classic ignition or fueling issue.
But the pattern didn’t match.
Step 1 – Identify When the Misfire Occurs
Misfire counters on the scan tool showed something unusual.
One cylinder accumulated misfires only during light cruise conditions.
Not idle.
Not heavy load.
This pattern is important.
Spark problems usually appear under load.
Fuel problems often appear across multiple conditions.
This suggested something mechanical.
Step 2 – Mechanical Testing
Compression test showed acceptable numbers.
However, compression tests can miss certain valvetrain problems.
Next step was observing valve lift.
After further inspection, the problem was discovered.
Root Cause
One camshaft lobe had excessive wear.
At idle the valve still opened enough to maintain combustion.
At high RPM the valve followed the cam profile due to inertia.
But at mid-range RPM the valve lift was insufficient.
That reduced airflow just enough to cause a misfire.
Diagnostic Takeaway
When misfires occur only in specific operating ranges, consider:
• valve train wear
• cam lobe damage
• valve spring issues
• restricted airflow
Not every misfire is spark or fuel.
Sometimes the problem is breathing.
Question for the group
What’s the strangest root cause you’ve found behind a misfire?
Broken valve spring?
Worn cam?
Collapsed lifter?
These weird ones are always the best learning experiences
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Case Study- Intermittent Misfire - 2012 Dodge Charger SRT8
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