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Rough idle on 2017 CX-9, time for new spark plugs?

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Amanda S. · rep 0 · May 22, 4:51 AM · 1 replies · solved · Question

Vehicle: 2017 Mazda CX-9 Grand Touring

Hello forum,

My 2017 Mazda CX-9 Grand Touring, with about 85,000 miles on the odometer, has started to develop a rough idle. It's not constant, but I notice it more when I'm stopped at a light. I also feel like my fuel economy has dipped a bit recently, though I haven't tracked it precisely.

Could this be a sign that it's time to replace the spark plugs? What's the recommended interval for the 2.5T engine in these CX-9s? Any other things I should check if it's not the plugs?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:51 AM

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ClearTheCode admin answer:

A rough idle and decreased fuel economy at 85,000 miles on your 2017 Mazda CX-9 are indeed strong indicators that your spark plugs might be due for replacement. The 2.5T engine typically uses long-life iridium spark plugs, but even these have a service life.

Spark Plug Service Interval:

  • For the 2.5T engine in the CX-9, Mazda generally recommends inspecting or replacing spark plugs around the 75,000 to 100,000-mile mark, depending on driving conditions. Your mileage falls right within this window.

Symptoms of Worn Spark Plugs:

  • Rough Idle: One of the most common signs, as misfires can occur.
  • Reduced Fuel Economy: Inefficient combustion uses more fuel.
  • Engine Misfires: Can feel like a hesitation or stumble, especially under acceleration.
  • Difficulty Starting: Though less common with modern ignition systems.
  • Check Engine Light: If misfires are severe enough, the engine control unit (ECU) will trigger a 'Check Engine' light and store diagnostic trouble codes (DTCs).

Relevant OBD Codes:

  • P0301 - P0304: These codes indicate a misfire in a specific cylinder (e.g., P0301 for cylinder 1, P0302 for cylinder 2, etc.). If you have a 'Check Engine' light, reading these codes with an OBD-II scanner can pinpoint which cylinder is having issues.
  • P0300: Random/Multiple Cylinder Misfire Detected.

Other things to check:

  • Ignition Coils: Worn ignition coils can also cause similar symptoms. Often, if one plug is bad, the corresponding coil might also be failing.
  • Air Filter: A dirty engine air filter can restrict airflow, affecting performance.
  • Fuel System: Clogged fuel injectors or a failing fuel pump could also contribute to rough idle.

When to see a shop:

  • While spark plug replacement can be a DIY task, it requires proper tools (torque wrench, spark plug socket) and care to avoid cross-threading or breaking a plug. If you're not comfortable, a professional can handle it.
  • If you have a 'Check Engine' light, it's best to get the codes read and diagnosed by a professional to ensure the correct component is replaced.
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