why does my Toyota Highlander Third Generation (2014-2019) have a rough start when cold?

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a flat surface and the engine is cool.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Check and Replace Spark Plugs

• Remove the engine cover if applicable.
• Disconnect each ignition coil connector.
• Remove the ignition coils using a socket set.
• Remove old spark plugs with a spark plug socket.
• Inspect spark plugs for wear; replace with OEM equivalents if necessary (typically NGK or Denso).
• Torque new spark plugs to manufacturer specifications (usually around 13 lb-ft).

3. Inspect and Replace Fuel Filter

• Locate the fuel filter (may be integrated in the fuel pump module).
• Relieve fuel system pressure by removing the fuel pump fuse and running the engine until it stalls.
• Disconnect fuel lines from the filter (use caution; fuel may spill).
• Replace the filter with a new one, ensuring proper flow direction.
• Reconnect fuel lines and reinstall the fuel pump fuse.

4. Check Coolant Temperature Sensor

• Locate the coolant temperature sensor on the engine.
• Disconnect the wiring harness and remove the sensor using a wrench.
• Test the sensor's resistance with a multimeter; replace if out of specification.
• Install a new sensor with a small amount of thread sealant, if necessary, and reconnect the harness.

5. Reconnect Battery and Test

• Reconnect the negative battery terminal.
• Start the engine and let it idle until it reaches operating temperature, monitoring for any roughness.