Backfire from the exhaust after it warms up to the operating temperature

Hi,

Have you retrieved diagnostic trouble codes? Also, what you described sounds like a bad crankshaft position sensor. They can fail when they get hot and then start working when it cools off. Was that replaced?

If you haven't already, check to see if there are trouble codes. Here is a quick video showing how it's done:

https://youtu.be/InIlnsjOVFA

Also, here are directions for replacing the crankshaft position sensor (if you haven't already done it.

1997 Chevy Truck K 1500 Truck 4WD V8-5.7L VIN R
Crankshaft Position 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 Sensor Replacement
CRANKSHAFT POSITION SENSOR REPLACEMENT

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REMOVAL PROCEDURE
1. Remove the electrical connector.
2. Remove the sensor hold down bolt.
3. Remove the sensor from the timing cover.

NOTE: When installing or removing a crankshaft position (CKP) sensor make sure the sensor is fully seated and held stationary in the front cover before torquing the hold down bolt into the front cover. A sensor which is not seated may result in erratic operation and lead to the setting of false codes.

4. Inspect the sensor O-ring for wear, cracks or leakage. Replace if necessary. Lube the new O-ring with clean engine oil before installing.

INSTALLATION PROCEDURE

NOTE: Make certain that the Crankshaft Position (CKP) sensor mounting surfaces are clean and free of burrs before installing the CKP sensor.

1. Install the sensor into the timing cover.
2. Install the sensor hold down bolt.

Tighten
Tighten the hold down bolt to 8 Nm (71 lb in).

3. Install the electrical connector.

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Let me know if that helps or if it was already done.

Take care,
Joe