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2016 IS 300 Rough Idle - No CEL Yet

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Mike T. · rep 0 · May 25, 3:24 AM · 1 replies · solved · Question

Vehicle: 2016 Lexus IS 300 F Sport AWD

Hey everyone, I've got a weird issue with my 2016 Lexus IS 300 (AWD, F Sport). For the past week or so, I've noticed a rough idle, especially when the car is warmed up and I'm sitting at a stoplight. It's not violent, but I can definitely feel a slight shake through the steering wheel and seat.

There's no check engine light on yet, which is making it harder to diagnose. The car drives fine otherwise, and acceleration seems normal. I've got about 75,000 miles on it. Any ideas on what might be causing this? Could it be spark plugs, even without a CEL?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:24 AM

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ClearTheCode admin answer: A rough idle on your 2016 Lexus IS 300 without a Check Engine Light (CEL) can be frustrating to diagnose, but it's a common symptom for several underlying issues. The lack of a CEL doesn't mean there isn't a problem; sometimes a fault isn't severe enough to trigger a code immediately.

Here are the common causes and steps to diagnose:

  1. Spark Plugs: At 75,000 miles, your spark plugs are likely due for replacement if they haven't been changed. Worn spark plugs can lead to inefficient combustion and a rough idle. Even if they're not completely failed, degraded plugs can cause subtle misfires not yet severe enough for a CEL.
  2. Ignition Coils: A failing ignition coil can cause intermittent misfires, especially at idle. While a complete failure usually triggers a CEL, a weak coil might only cause noticeable symptoms under specific conditions.
  3. Fuel Injectors: Clogged or dirty fuel injectors can lead to an improper fuel spray pattern, causing one or more cylinders to run lean or rich, resulting in a rough idle. Fuel system cleaners can sometimes help, but severe clogs may require professional cleaning or replacement.
  4. Mass Airflow (MAF) Sensor: A dirty or faulty MAF sensor can send incorrect air intake readings to the ECU, leading to an incorrect fuel-air mixture and a rough idle.
  5. Vacuum Leaks: Leaks in vacuum lines or intake manifold gaskets can introduce unmetered air into the engine, disrupting the fuel-air mixture and causing a rough idle.
  6. Throttle Body: A dirty or carbon-fouled throttle body can restrict airflow, especially at idle, leading to an uneven engine speed.

When to see a shop:

  • Persistent Symptoms: If the rough idle persists or worsens, it's wise to have a shop diagnose it. Even without a CEL, a professional scan tool can often detect pending codes or show live data that points to the issue.
  • Preventative Maintenance: Consider starting with spark plug replacement if they are due. If that doesn't resolve it, a mechanic can perform a comprehensive diagnostic, including checking vacuum, fuel pressure, and live sensor data.
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