Symptom Medium Severity Safe to Drive: Yes At Idle

Why Your Car Shakes Slightly at Idle (Causes & Fixes)

A slight vibration at idle often points to common engine, ignition, fuel, air, or mount issues; quick checks can help identify the underlying cause.

Potential Causes

Worn or Faulty Spark Plugs High Probability

Worn spark plugs cause misfires, leading to uneven combustion and vibration at idle.

Damaged or Weak Engine (Motor) Mounts High Probability

Worn engine mounts fail to dampen normal engine vibrations, transmitting them into the cabin at idle.

Vacuum Leak Medium Probability

Unmetered air entering through a cracked hose disrupts air-fuel balance, causing rough idle vibration.

Dirty or Clogged Fuel Injectors Medium Probability

Fuel delivery imbalance from clogged injectors causes uneven combustion and idle shaking.

Idle Air Control Valve or Throttle Body Dirt Low Probability

Carbon buildup affects air regulation at idle, leading to unstable idle and vibration.

Diagnostic Steps

  1. Check for Engine Codes

    Use an OBD-II scanner to check for misfire or lean condition codes.

  2. Inspect Spark Plugs

    Remove and inspect spark plugs for wear or fouling.

  3. Check Engine Mounts

    Visually inspect engine mounts for cracks or deterioration.

  4. Inspect Vacuum Hoses

    Check for cracked or loose vacuum hoses causing unmetered air leaks.

  5. Clean Throttle Body

    Clean carbon buildup in throttle body and idle air control area.

DIY Fixes

Replace Spark Plugs Beginner
Estimated Cost: $20 - $100

Clean Throttle Body Intermediate
Estimated Cost: $10 - $50

Inspect and Replace Vacuum Hoses Intermediate
Estimated Cost: $5 - $50

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 cylinder misfires causing rough idle vibration.

Frequently Asked Questions

Is slight shaking at idle normal?

A slight vibration can be normal, but noticeable shaking suggests an underlying issue like misfires or mounts.

Can worn spark plugs cause idle vibration?

Yes, worn plugs cause misfires that often show as rough idle vibration.

Is it expensive to fix rough idle issues?

Simple fixes like spark plug replacement or cleaning are relatively inexpensive; major repairs like mounts cost more.

Commonly Related Terms

car vibrates at idle rough idle vibration engine idle shaking shaky idle when stopped

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