Loss of power and backfires when warm outside

Hi,

The idea that it runs properly when you allow it to cool off leads me to think it could be related to the crankshaft position sensor. When they start going bad, they are adversely affected by heat. Take a look through this link and let me know if it seems to mirror what you experience:

https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor

When you get a backfire, oftentimes it is related to a fuel-related issue (too much fuel), but that wouldn't only happen when it's hot. That is what is leading me in the direction I mentioned.

If you find the sensor is bad, here is a link that shows in general how one is replaced. It will help as a guide.

https://www.2carpros.com/articles/crankshaft-angle-sensor-replacement

Here are directions specific to your vehicle. The pics below correlate with the directions. You will need to remove the starter to access the sensor, so make sure the battery is disconnected before you start.

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2003 GMC Truck K 1500 Yukon XL 4WD V8-5.3L VIN T
Crankshaft Position (CKP) Sensor Replacement
Vehicle Powertrain Management Sensors and Switches - Powertrain Management Sensors and Switches - Computers and Control Systems Crankshaft Position Sensor Service and Repair Procedures Crankshaft Position (CKP) Sensor Replacement
CRANKSHAFT POSITION (CKP) SENSOR REPLACEMENT
Crankshaft Position (CKP) Sensor Replacement
REMOVAL PROCEDURE

IMPORTANT: Perform the CKP System Variation Learn Procedure whenever the crankshaft position sensor is removed or replaced.

pic 1

1. Remove the starter.
2. Disconnect the crankshaft position (CKP) sensor electrical connector.

Crankshaft Position Sensor (CKP)

pic 2

3. Clean the area around the CKP sensor before removal in order to avoid debris from entering the engine.
4. Remove the CKP sensor bolt.
5. Remove the CKP sensor.

Crankshaft Position Sensor (CKP)

pic 3

INSTALLATION PROCEDURE
1. Install the CKP sensor.

NOTE: Refer to Fastener Notice in Service Precautions See: Powertrain Management > Vehicle Damage Warnings > Fastener Notice.

2. Install the CKP sensor bolt.

Tighten
Tighten the bolt to .................................25 N.m (18 lb ft).

pic 4

3. Connect the CKP sensor electrical connector.
4. Install the starter.
5. Perform the CKP system variation learn procedure. See: Vehicle > Programming and Relearning

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You will not need to perform the relearn for it to start. However, if you experience drivability issues, you will need to do it. Here are the relearn procedures.

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2003 GMC Truck K 1500 Yukon XL 4WD V8-5.3L VIN T
Crankshaft Position (CKP) - Variation Learn Procedure
Vehicle Testing and Inspection Programming and Relearning Crankshaft Position (CKP) - Variation Learn Procedure
CRANKSHAFT POSITION (CKP) - VARIATION LEARN PROCEDURE
CKP System Variation Learn Procedure
1. Install a scan tool.

2. Monitor the powertrain control module for DTCs with a scan tool. If other DTCs are set, except DTC P0315, refer to Diagnostic Trouble Code (DTC) List for the applicable DTC that set.

3. Select the crankshaft position (CKP) variation learn procedure with a scan tool.

4. The scan tool instructs you to perform the following:
- Accelerate to wide open throttle (WOT).
- Release the throttle when fuel cut-off occurs.
- Observe the fuel cut-off for the engine that you are performing the learn procedure on.
- The engine should not accelerate beyond the calibrated RPM value.
- Release the throttle immediately if the value is exceeded.
- Block the drive wheels.
- Set the parking brake.
- DO NOT apply the brake pedal.
- Cycle the ignition from OFF to ON.
- Apply and hold the brake pedal.
- Start and idle the engine.
- Turn the A/C OFF.
- The vehicle must remain in Park or Neutral.
- The scan tool monitors certain component signals to determine if all the conditions are met to continue with the procedure. The scan tool only displays the condition that inhibits the procedure. The scan tool monitors the following components:
- Crankshaft position (CKP) sensors activity-If there is a CKP sensor condition, refer to the applicable DTC that set.
- Camshaft position (CMP) sensor activity-If there is a CMP sensor condition, refer to the applicable DTC that set.
- Engine coolant temperature (ECT)-If the ECT is not warm enough, idle the engine until the engine coolant temperature reaches the correct temperature.

5. Enable the CKP system variation learn procedure with a scan tool.
6. Accelerate to WOT.

IMPORTANT: While the learn procedure is in progress, release the throttle immediately when the engine starts to decelerate. The engine control is returned to the operator and the engine responds to throttle position after the learn procedure is complete.

7. Release when the fuel cut-off occurs.
8. The scan tool displays Learn Status: Learned this ignition.
If the scan tool indicates that DTC P0315 ran and passed, the CKP variation learn procedure is complete.
If the scan tool indicates DTC P0315 failed or did not run, refer to DTC P0315.
If any other DTCs set, refer to Diagnostic Trouble Code (DTC) List for the applicable DTC that set.

9. Turn OFF the ignition for 30 seconds after the learn procedure is completed successfully.
10. The CKP system variation learn procedure is also required when the following service procedures have been performed, regardless of whether DTC P0315 is set:
- An engine replacement
- A PCM replacement
- A harmonic balancer replacement
- A crankshaft replacement
- Any engine repairs which disturb the CKP sensor relationship

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Let me know if this helps or if you have other questions.

Take care and God Bless,

Joe