Rough idle and misfires on my 2017 SQ5 Premium Plus

ClearTheCode admin answer:

Experiencing a rough idle, hesitation, and misfire codes like P0300 (Random/Multiple Cylinder Misfire Detected) and P0304 (Cylinder 4 Misfire Detected) on your 2017 Audi SQ5 at 85,000 miles strongly suggests an ignition system issue.

Here’s a breakdown of what to check and potential causes:

1. Confirm Misfire Location:
   • Since you have P0304, focus diagnostics on cylinder 4 initially. If P0300 is also present, it indicates the issue might be broader or intermittent.
2. Inspect Ignition Coils:
   • Ignition coils are a common failure point. You can try swapping the ignition coil from cylinder 4 to another cylinder (e.g., cylinder 1) and see if the misfire code follows the coil (e.g., P0301 appears). This is a good diagnostic step.
3. Check Spark Plugs:
   • Spark plugs are wear items and should be replaced according to Audi's service schedule. Old or fouled plugs can cause misfires. Inspect the spark plug from cylinder 4 for signs of wear, carbon buildup, or damage.
4. Fuel Injectors:
   • Less common, but a clogged or failing fuel injector in cylinder 4 could also cause a misfire. This usually requires more advanced diagnostic tools to confirm.
5. Vacuum Leaks:
   • A vacuum leak could lead to a lean condition and misfires, especially at idle. Listen for hissing sounds and inspect vacuum lines for cracks.

Common Causes:

• Failing Ignition Coils: The most frequent cause of misfires in these engines, especially as they age.
• Worn Spark Plugs: Beyond their service life, plugs become less efficient.
• Carbon Buildup: Direct injection engines like the SQ5's 3.0T are prone to carbon buildup on intake valves, which can restrict airflow and cause misfires. This usually manifests as a broader performance issue rather than a single cylinder misfire.

When to See a Shop:

• As soon as possible: If the check engine light is flashing, it indicates a severe misfire that could be damaging the catalytic converter. Drive as little as possible.
• If DIY diagnostics don't resolve it: If swapping coils or replacing plugs doesn't fix the issue, or if you suspect carbon buildup or a fuel system problem, a professional diagnosis is needed. An Audi specialist can perform further tests like compression checks, fuel pressure tests, and injector flow tests to accurately diagnose the root cause.