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Base Engine Misfire with Abnormal Internal Lower Engine Noises

WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.
Cause Correction
Abnormalities, such as severe cracking, bumps or missing areas in the accessory drive belt
Abnormalities in the accessory drive system and/or components may cause engine RPM variations, noises similar to a faulty lower engine and also lead to a misfire condition. A misfire code may be present without an actual misfire condition.		 Replace the drive belt. Drive Belt Replacement - Accessory 
Worn, damaged, or mis-aligned accessory drive components or excessive pulley runout
A misfire code may be present without an actual misfire condition.		 Inspect the components and repair or replace, as required.
Loose or improperly installed engine flywheel, flex plate or crankshaft balancer
A misfire code may be present without an actual misfire condition.		 Repair or replace the engine flywheel, flex plate, and/or balancer, as required. Automatic Transmission Flex Plate Removal , and Automatic Transmission Flex Plate Installation , and/or Crankshaft Balancer Removal , and Crankshaft Balancer Installation .
Worn piston rings
Oil consumption may or may not cause the engine to misfire.
  1. Inspect the spark plugs for oil deposits. Spark Plug Inspection
  2. Inspect the cylinders for a loss of compression. Engine Compression Test 
  3. Perform cylinder leak down and compression testing to determine the cause. Cylinder Leakage Test .
  4. Repair or replace, as required.
Worn crankshaft thrust bearings
Severely worn thrust surfaces on the crankshaft and/or thrust bearing may permit fore and aft movement of the crankshaft and create a DTC without an actual misfire condition.		 Replace the crankshaft and bearings, as required.
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