GMC Sierra T1XX (2019-Present) rough idle when cold start

Fix rough idle during cold starts in GMC Sierra T1XX. Step-by-step diagnostic and repair guide for engine management issues.

Solution

1. Preparation

• Gather tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Allow the engine to cool completely before starting work.

2. Inspect and Replace Spark Plugs

• Tools Required: Spark plug socket, ratchet, torque wrench.
• Remove spark plug wires or coils.
• Use a spark plug socket to remove old spark plugs.
• Inspect the spark plugs for wear or fouling.
• Install new spark plugs, setting the torque to manufacturer specifications (typically around 15 ft-lbs).
• Reconnect spark plug wires or coils.

3. Clean Throttle Body

• Tools Required: Throttle body cleaner, soft cloth, screwdriver.
• Remove the air intake duct to access the throttle body.
• Spray throttle body cleaner onto a clean cloth and wipe down the throttle body inside and out.
• Ensure the throttle plate opens and closes freely.
• Reinstall the air intake duct securely.

4. Replace Coolant Temperature Sensor (if faulty)

• Tools Required: Socket set, pliers.
• Locate the coolant temperature sensor (usually near the thermostat housing).
• Disconnect the electrical connector and remove the sensor.
• Install the new sensor, ensuring it is properly sealed.
• Reconnect the electrical connector.

5. Check Fuel Pressure

• Tools Required: Fuel pressure gauge.
• Locate the fuel pressure test port on the fuel rail.
• Connect the fuel pressure gauge and turn the ignition to the "On" position without starting the engine.
• Check the fuel pressure reading against manufacturer specifications (typically around 55-62 psi).
• If pressure is low, inspect the fuel pump and filter for issues.