how to fix the rough idle in my Volkswagen Passat B8 (2015-2022) when it's cold?

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the battery to ensure safety while working on electrical components.

2. Inspect and Replace Spark Plugs

• Remove the ignition coil cover using a socket set.
• Disconnect the electrical connectors from the ignition coils.
• Remove the ignition coils and spark plugs using a spark plug socket.
• Inspect spark plugs for wear; replace with OEM-spec plugs if damaged.
• Reinstall new spark plugs and ignition coils, ensuring proper torque (typically 15-20 Nm).

3. Check and Adjust Fuel Pressure

• Connect a fuel pressure gauge to the fuel rail Schrader valve.
• Turn the ignition on (do not start the engine) and observe the fuel pressure readings; it should meet specifications (usually around 50-60 psi).
• If the pressure is low, inspect the fuel pump and filter, replacing them if necessary.

4. Clean the Throttle Body

• Remove the air intake duct connected to the throttle body.
• Use throttle body cleaner and a soft cloth to clean any carbon buildup from the throttle plate and surrounding areas.
• Reassemble the air intake duct, ensuring it is properly secured.

5. Replace the Coolant Temperature Sensor (if faulty)

• Locate the coolant temperature sensor (often near the thermostat housing).
• Disconnect the electrical connector and remove the sensor using a wrench.
• Install the new sensor, ensuring appropriate torque (usually around 20 Nm), and reconnect the electrical connector.