Engine
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    Why Does My Car Idle Rough?

    Typical repair cost: $0–$1,200

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    Common Symptoms

    RPM bouncing at idle
    Engine shaking in park
    Stalls at stops
    Check engine light
    Smell of unburnt fuel

    Most Likely Causes

    1

    Vacuum leak

    Cracked intake hose, PCV, or gasket. Look for P0171/P0174 lean codes.

    2

    Dirty or failing throttle body

    Carbon deposits prevent smooth idle airflow.

    3

    Bad spark plugs or coils

    Single misfire pulls RPM around — check P0301-P0312.

    4

    Failing MAF sensor

    Dirty MAF reports wrong airflow — try cleaning with MAF-safe spray.

    5

    Carbon-loaded GDI valves

    Common on direct-injection engines after 60-80k miles.

    How a Shop Diagnoses It

    1. 1.Scan for misfire and lean/rich codes.
    2. 2.Smoke-test intake for vacuum leaks.
    3. 3.Inspect/clean throttle body and MAF.
    4. 4.Check fuel trim values (LTFT > 10% = problem).

    When to see a professional

    If the CEL is flashing, stop driving — flashing CEL during misfire destroys catalytic converters.

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    $80–$2000 typical repair
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    $50–$8000 typical repair

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