Symptom Medium Severity Safe to Drive: Caution At Idle

Why Your Car Shakes at Idle (Engine Rough Idle Causes)

Car shaking when idling is usually due to engine performance, fuel delivery, or mount issues. Common causes include misfires, vacuum leaks, and worn engine mounts disrupting smooth idle operation.

Potential Causes

Worn or Faulty Spark Plugs/Ignition High Probability

Faulty spark plugs or ignition components cause misfires and incomplete combustion leading to rough idle vibrations. Spark plugs ignite fuel-air mix; wear disrupts combustion balance.

Vacuum Leak Medium Probability

Unmetered air entering engine disturbs air-fuel mixture causing lean conditions and rough idle vibration.

Dirty/Clogged Fuel Injectors Medium Probability

Fuel injectors that deliver uneven fuel result in irregular combustion especially at idle.

Damaged Engine/Motor Mounts Medium Probability

Worn mounts fail to damp engine vibration, transmitting shakes into cabin at idle.

Faulty Idle Air Control Valve or MAF Sensor Low Probability

Idle airflow issues cause unstable idle speed and vibration.

Diagnostic Steps

  1. Check Engine Light & Read Codes

    Use OBD-II scanner to retrieve DTCs and note misfire or fuel/air ratio codes.

  2. Visual Inspection of Hoses & Plugs

    Inspect vacuum hoses, spark plugs, and ignition wiring for wear or damage.

  3. Test Engine Mounts

    Observe vibrations in gear vs neutral; inspect mounts for cracks or degradation.

  4. Fuel Injector & Airflow Check

    Clean or test fuel injectors and throttle body; check air filter and MAF readings.

DIY Fixes

Replace Spark Plugs Intermediate
Estimated Cost: $50 - $120

Inspect & Replace Vacuum Hoses Intermediate
Estimated Cost: $10 - $50

Clean Throttle Body & Fuel Injectors Medium
Estimated Cost: $20 - $100

OBD-II Live Data Analysis

Use a scan tool to monitor these parameters. Comparing live values against the normal range can help identify the root cause.

PID

Long Term Fuel Trim (LTFT)

Normal Range +/- 10%
Abnormal Condition > +15% (Lean)
Technical Insight: Indicates unmetered air (vacuum leak) causing rough idle even if no P0171 code is set.
PID

Misfire Counts

Normal Range 0
Abnormal Condition > 0
Technical Insight: Shows active cylinder misfires contributing to idle shakes.

Frequently Asked Questions

Is it expensive to fix rough idle?

Costs vary by cause; spark plug replacement is relatively inexpensive, but engine mount or injector issues cost more.

Can I drive with a shaky idle?

Short drives are generally safe, but avoid long trips until you diagnose as misfires can damage catalytic converter.

Why does idle shake get worse when accessories run?

Extra load can unmask weak idle control or mounts, increasing vibration.

Commonly Related Terms

Car shakes while idling Car vibrates at idle Engine shudders when stopped Rough idle vibration

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