Symptom Medium Severity Safe to Drive: Caution At Idle

Why Your Car Runs Rough Only When Stopped (Causes & Fixes)

Car runs rough or shakes only when stopped due to idle control, fuel, ignition, or sensor issues that disturb smooth combustion at idle speed. Learn likely causes and fixes.

Potential Causes

Dirty/Clogged Fuel Injectors Medium Probability

Uneven fuel delivery leads to irregular combustion at idle, rough feeling when stopped

Vacuum Leak Medium Probability

Unmetered air causes lean condition and unstable idle

Faulty Idle Air Control Valve/Throttle Body High Probability

Improper idle airflow regulation causes unstable idle speed when stopped

Ignition System Issues (Plugs/Coils) Medium Probability

Weak spark causes misfires at idle under low load conditions

Mass Air Flow or MAP Sensor Fault Low Probability

Incorrect air measurement leads to wrong fuel trim affecting idle

Diagnostic Steps

  1. Check for Stored Codes

    Use OBD-II scanner to read stored codes and freeze frame data

  2. Inspect and Clean Throttle Body

    Remove carbon buildup and inspect idle control passages

  3. Inspect Vacuum Lines

    Check hoses for cracks/disconnections causing unmetered air

  4. Check Ignition Components

    Inspect spark plugs and coils for wear or misfire indicators

  5. Fuel System Service

    Clean fuel injectors and check fuel pressure

DIY Fixes

Clean Throttle Body & Idle Air Control Intermediate
Estimated Cost: $10 - $40

Replace Spark Plugs Beginner
Estimated Cost: $20 - $80

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 vacuum leak or fuel delivery issue affecting idle
PID

Short Term Fuel Trim (STFT)

Normal Range +/- 10%
Abnormal Condition > +/- 15%
Technical Insight: Shows immediate air/fuel imbalance often seen at idle

Frequently Asked Questions

Why does my car run rough only when stopped?

A rough idle when stopped can be caused by issues such as dirty or clogged fuel injectors, a vacuum leak, or a faulty idle air control valve or throttle body.

What are the potential causes of a rough idle in my car?

Common causes include dirty or clogged fuel injectors, vacuum leaks, and problems with the idle air control valve or throttle body.

How can I diagnose why my car runs rough at a stop?

Use an OBD-II scanner to check for stored codes and freeze frame data. Additionally, inspect for carbon buildup and check hoses for cracks or disconnections that may cause unmetered air.

What DIY fixes can I try for a rough idle when stopped?

You can clean the throttle body and idle air control valve, and consider replacing spark plugs to improve engine performance.

Is it safe to drive my car if it runs rough when stopped?

While it may be safe for short distances, a rough idle can indicate underlying issues that could worsen over time. It's advisable to diagnose and address the problem as soon as possible.

Commonly Related Terms

Rough idle at stop Engine shakes at idle Rough idle when stationary Idling rough at traffic light

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