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Rough Idle and Misfire on 2017 Mazda 6 Touring

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Christine P. · rep 0 · May 22, 4:48 AM · 1 replies · solved · Question

Vehicle: 2017 Mazda 6 Touring

My 2017 Mazda 6 Touring, with the 2.5L engine, has recently developed a noticeable rough idle. It's particularly bad when the engine is cold, and sometimes it feels like it's on the verge of stalling. I can feel a distinct vibration and shake throughout the car when it's idling.

So far, I haven't seen a check engine light come on, which is a bit puzzling. The car has about 75,000 miles on it, and I've kept up with regular maintenance, though I haven't changed the spark plugs yet.

Could this be due to old spark plugs, a failing ignition coil, or something else like a vacuum leak? What should I be looking for or testing to narrow down the cause of this rough idle and potential misfire?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:48 AM

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ClearTheCode admin answer:

A rough idle and potential misfire without a check engine light can be frustrating, but it's a common issue with several possible culprits. Given your mileage, it's a good time to consider some maintenance items.

Here are the most common causes for a rough idle and what to check:

  1. Spark Plugs: At 75,000 miles, your spark plugs are likely due for replacement. Worn spark plugs can lead to weak or inconsistent spark, causing misfires and a rough idle, especially when cold. This is a relatively inexpensive and often effective first step.
  2. Ignition Coils: A failing ignition coil can cause a cylinder to misfire. While a complete failure usually triggers a CEL, an intermittent or weak coil might cause a rough idle without a code initially.
  3. Fuel Injectors: Clogged or faulty fuel injectors can lead to an improper fuel-air mixture in one or more cylinders, resulting in a rough idle. Fuel injector cleaner can sometimes help, but a faulty injector may need replacement.
  4. Vacuum Leaks: Leaks in the intake manifold, vacuum lines, or PCV (Positive Crankcase Ventilation) system can introduce unmetered air into the engine, disrupting the fuel-air ratio and causing a rough idle. Listen for hissing sounds under the hood.
  5. Mass Airflow (MAF) Sensor: A dirty or failing MAF sensor can send incorrect air intake readings to the ECU, leading to an incorrect fuel mixture and rough idle. Cleaning it with MAF cleaner can sometimes resolve this.
  6. Throttle Body: A dirty throttle body can restrict airflow and cause an inconsistent idle. Cleaning it can often improve idle quality.

When to see a shop:

  • If replacing spark plugs doesn't resolve the issue.
  • If you're unable to locate any vacuum leaks or if cleaning the MAF sensor and throttle body doesn't help.
  • If the rough idle worsens, or if a check engine light eventually comes on (which will provide specific misfire codes, e.g., P0301 for cylinder 1 misfire, etc.). A professional diagnostic scan can help pinpoint the exact cylinder or component causing the misfire by monitoring live data.
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