Symptom Medium Severity Safe to Drive: Caution At Idle

Why Your Car Sometimes Lurches at Traffic Lights (And What It Means)

Experiencing lurching or jerking when stopped at traffic lights often points to issues with idle stability. Common causes include ignition, fuel, air intake or idle control faults that disrupt smooth engine operation at idle.

Potential Causes

Dirty or Faulty Idle Air Control Valve / Throttle Body High Probability

Build-up of grime affects idle air flow, leading to unstable idle and lurching at idle.

Worn Spark Plugs or Ignition Issues Medium Probability

Faulty ignition components cause misfires and uneven combustion, leading to jerking at lights. ()

Vacuum Leaks Medium Probability

Unmetered air alters air/fuel mixture causing rough idle and lurching.

Fuel Delivery Problems Low Probability

Clogged fuel injectors or weak fuel pump can starve engine at idle. ()

Diagnostic Steps

  1. Check for Engine Codes

    Scan OBD-II for misfire, idle control, MAF or fuel trim codes

  2. Inspect Spark Plugs and Ignition

    Check spark plugs and coils for wear or damage

  3. Clean Throttle Body and IAC

    Remove and clean throttle body and idle air control valve

  4. Check Vacuum Lines

    Inspect vacuum hoses for leaks or cracks

  5. Verify Fuel Delivery

    Check fuel pressure and injector operation

DIY Fixes

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

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: High LTFT indicates vacuum leak or unmetered air disrupting idle
PID

Commanded Idle Speed

Normal Range ~650–900 RPM
Abnormal Condition Fluctuating wildly at idle
Technical Insight: Suggests idle control or air delivery malfunction

Frequently Asked Questions

Is it dangerous if my car lurches at lights?

Occasional minor lurching may not be dangerous, but frequent jerking or stalling increases risk of stalling in traffic.

Can I drive with this issue?

You can drive cautiously, but have it checked soon to avoid worsening performance or stalling.

Will cleaning throttle body fix it?

Often cleaning throttle body and idle control components improves idle stability if dirt build-up was the cause.

Commonly Related Terms

Car jerks when idling at stop light Car lurches at traffic lights Engine surges at idle Car shakes at lights

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