my GMC Sierra T1XX (2019-Present) has a rough idle when cold, what could be wrong?

Learn how to diagnose and fix rough idle issues in GMC Sierra T1XX (2019-Present). Step-by-step guide with symptoms, diagnostic steps, and repair instructions.

Solution

1. Preparation

• Gather all necessary tools and parts.
• Ensure the vehicle is parked on a level surface with the engine off and cool.
• Disconnect the battery to prevent any electrical shorts while working.

2. Inspect and Replace Spark Plugs

• Sub-steps:
  1. Remove the engine cover (if applicable) using a socket set.
  2. Disconnect the ignition coils from the spark plugs.
  3. Use a spark plug socket to remove the old spark plugs.
  4. Inspect the old spark plugs for signs of wear or fouling.
  5. Install new spark plugs, ensuring the proper gap (consult manufacturer specifications).
  6. Reconnect the ignition coils and replace the engine cover.

3. Clean Throttle Body

• Sub-steps:
  1. Remove the air intake duct to access the throttle body.
  2. Use throttle body cleaner and a soft cloth to clean the inside of the throttle body.
  3. Ensure the throttle plate moves freely; manually open and close it to check for obstructions.
  4. Reassemble the air intake duct securely.

4. Check and Replace Fuel Filter (if applicable)

• Sub-steps:
  1. Locate the fuel filter (may be integrated with the fuel pump in some models).
  2. Relieve fuel system pressure using the fuel pressure relief valve.
  3. Disconnect the fuel lines from the filter with a fuel line disconnect tool.
  4. Replace the old filter with a new one, ensuring correct flow direction.
  5. Reconnect the fuel lines and check for leaks.

5. Verify Coolant Temperature Sensor

• Sub-steps:
  1. Locate the ECT sensor (usually near the thermostat housing).
  2. Disconnect the wiring harness and inspect for damage.
  3. Use a multimeter to test the resistance of the sensor at different temperatures.
  4. Replace the sensor if readings are out of specified range.

Tools Required:

• OBD-II scanner
• Socket set
• Spark plug socket
• Throttle body cleaner
• Soft cloth
• Fuel pressure gauge
• Fuel line disconnect tool
• Multimeter

Parts Required:

• Spark plugs (specific to engine type)
• Fuel filter (if separate from the fuel pump)
• Engine coolant temperature sensor (if faulty)

Estimated Repair Time: 2-4 hours

Safety Warnings:

• Disconnect the battery before working on electrical systems.
• Relieve fuel pressure before disconnecting fuel lines to avoid fuel spray.