Symptom Medium Severity Safe to Drive: Caution At Idle

Why Your Car Runs Rough at Night (Causes & Fixes)

A car that runs rough specifically during nighttime often points to engine performance issues such as ignition, fuel delivery, or sensor problems that disrupt smooth combustion.

Potential Causes

Faulty Spark Plugs or Ignition Coils High Probability

Worn spark plugs or failing coils cause misfires and inconsistent combustion leading to rough engine operation at idle or low load conditions typical in nighttime driving.

Vacuum Leak Medium Probability

A vacuum leak disrupts the air?fuel ratio causing rough running, particularly at idle speeds when airflow disruptions are more pronounced.

Dirty Fuel Injectors or Clogged Fuel Filter Medium Probability

Restricted fuel delivery causes uneven combustion and rough engine performance.

Faulty Sensors (MAF, O2) Medium Probability

Sensors that misread air or oxygen levels can cause incorrect fuel metering, leading to rough running.

Idle Air Control or Throttle Body Issues Low Probability

Malfunctioning idle air control or carbon buildup in throttle body can cause uneven air delivery at idle, worsening rough running.

Diagnostic Steps

  1. Scan for OBD?II Codes

    Use an OBD?II scanner to retrieve any stored diagnostic trouble codes to identify misfires or sensor faults

  2. Inspect Ignition Components

    Check spark plugs, ignition coils, and wires for wear or damage and replace if necessary

  3. Check for Vacuum Leaks

    Perform a smoke test or visual inspection of vacuum hoses and intake for leaks

  4. Inspect Fuel System

    Check fuel filter, injectors, and fuel pressure to ensure proper fuel delivery

  5. Sensor Function Tests

    Test MAF and O2 sensors using live data or multimeter to confirm correct operation

DIY Fixes

Replace Spark Plugs Beginner
Estimated Cost: $30 - $100

Clean MAF Sensor Intermediate
Estimated Cost: $10 - $30

Replace Air Filter Beginner
Estimated Cost: $10 - $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 per cylinder
Technical Insight: Indicates ignition or fuel delivery issues causing rough running

Frequently Asked Questions

Why does my car run rough only at night?

Nighttime rough running may be noticed more due to cooler temperatures and idle conditions exposing ignition or sensor issues; underlying causes remain mechanical or electrical.

Can I drive my car if it runs rough at night?

You can cautiously drive short distances, but prolonged driving may cause further engine wear; get diagnostics soon.

Will rough idle trigger my check engine light?

Yes, rough running often triggers codes related to misfires or sensor failures which illuminate the check engine light.

Commonly Related Terms

Engine runs rough at night Car rough idle at night Nighttime engine vibration Rough running after dark

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