Symptom Medium Severity Safe to Drive: Caution At Idle

Why Your Car Feels Uneven at Stop Signs (Rough Idle Diagnosis)

A car that feels uneven or shakes at stop signs is usually experiencing a rough idle, often caused by issues in ignition, fuel delivery, or airflow systems leading to uneven engine operation at low RPMs.

Potential Causes

Worn or fouled spark plugs High Probability

Spark plugs that are worn or fouled can cause misfires especially noticeable at idle, leading to vibration

Vacuum leak Medium Probability

Unmetered air from cracked or disconnected vacuum hoses disrupts air/fuel mixture causing rough idle

Dirty or clogged fuel injectors Medium Probability

Clogged fuel injectors cause uneven fuel delivery and misfires at idle

Dirty throttle body or idle air control issue Medium Probability

Carbon buildup restricts airflow at idle, causing unstable engine speed and vibration

Worn engine mounts Low Probability

Engine mounts transfer engine vibration to chassis more when worn, making idle feel uneven

Diagnostic Steps

  1. Scan for Trouble Codes

    Use an OBD?II scanner to detect any misfire or lean codes

  2. Inspect Spark Plugs and Ignition

    Remove and inspect spark plugs and ignition coils for wear or damage

  3. Check for Vacuum Leaks

    Listen for hissing and inspect vacuum hoses for cracks or disconnection

  4. Clean Throttle Body and Air Intake

    Clean carbon buildup from throttle body and check air filter condition

  5. Assess Engine Mounts

    Visually inspect engine mounts for wear and excessive movement

DIY Fixes

Replace Spark Plugs Beginner
Estimated Cost: $50 - $150

Clean Throttle Body Intermediate
Estimated Cost: $20 - $60

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 potential vacuum leaks or air/fuel imbalance causing rough idle
PID

Misfire Counters

Normal Range 0
Abnormal Condition > 0
Technical Insight: Live misfire counts on individual cylinders help pinpoint ignition or fuel delivery issues

Frequently Asked Questions

Is it safe to drive with rough idle?

Mild rough idle may be okay for a short drive, but if engine stalls or vibrates severely, seek immediate inspection.

Can bad spark plugs cause uneven feeling at stops?

Yes, worn plugs can misfire especially at idle, causing the car to feel uneven at stop signs.

Will cleaning the throttle body help?

Yes, cleaning a dirty throttle body can improve idle smoothness by restoring proper airflow.

Commonly Related Terms

car shakes when stopped at stop signs vehicle vibrates at every stop rough idle at stop signs engine shakes when stopping

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