Engine
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    Why Is My Engine Knocking?

    Typical repair cost: $50–$8,000

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    Common Symptoms

    Metallic knock at idle
    Knock louder under load
    Deep thunk on cold start
    Pinging on acceleration
    Knock that goes away when warm

    Most Likely Causes

    1

    Spark knock (pre-ignition)

    Low-octane fuel, carbon buildup, or advanced timing — heard under load. Triggers knock-sensor codes like P0325.

    2

    Rod knock

    Bottom-end bearing failure. Deep, rhythmic — every other RPM. Engine teardown territory.

    3

    Piston slap

    Cold-start knock that quiets as the piston warms and expands. Often livable.

    4

    Lifter tick / failed AFM lifter

    Common on GM 5.3L V8s — collapsed lifter from active fuel management.

    5

    Worn timing chain

    Slap against guide — usually accompanied by P0008/P0016.

    How a Shop Diagnoses It

    1. 1.Use a mechanic's stethoscope to localize the knock to top end vs bottom end.
    2. 2.Pull plugs and inspect for detonation damage.
    3. 3.Scan for knock-sensor and timing codes.
    4. 4.Oil sample analysis if bottom-end suspected.

    When to see a professional

    Bottom-end knock means stop driving. Pinging under load can be temporarily fixed with higher octane until diagnosed.

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