GMC Repair: 2004 GMC 2500HD ENGINE MISFIRE, failure records, system variation

Question
GOOD EVENING TIM:
HAVING SCANNED MOST OF YOUR GMC PROBLEMS,  IT SEEMS I'VE HAD ALOT
OF THE SAME PROBLEMS ALSO.   HERE'S ONE MY DEALER MECHANICS HAVEN'T
QUITE FIGURED OUT YET, MAINLY DUE TO FINANCIAL CONCERNS.

MY  2004 GMC 2500 HD XLT,  STARTING SHOWING THE CHECK ENGINE LIGHT
WAY BACK WHEN IT WAS UNDER 30,000 MILES,   YET THE DEALERS
MECHANICS CLAIMED IT NEVER READ A CODE.    NOW, A FEW YEARS LATER,
CLOSING IN ON 80,000 MILES,  THE CHECK ENGINE LIGHT ALWAYS FLASHES
WHEN I GO OVER 50 MPH,  THEN STAYS ON FOR A FEW DAYS AFTERWARDS.

MY DEALER MECHANICS NOW READ AN ENGINE MIS-FIRE CODE "PO300",
MISFIRE ON CYLINDERS 1 & 6.  THEY  SWAPPED AROUND THE COILS, PLUGS,
WIRES, REMOVED INJECTION RAILS AND SWAPPED AROUND ( WHATEVER THAT
MEANS ), TESTED FOR LEAKS, ( NONE FOUND ),   THEN NO FARTHER ACTION.

DEALER SAYS IT COULD COST UP TO $2500 TO GO INTO THE ENGINE TO
LOOK AT THE CYLINDERS.     ANY SUGGESTIONS WOULD BE GREATLY
APPRECIATED.!

RANDY S.

Answer
Important: You must perform the crankshaft position (CKP) system variation learn procedure before proceeding with this diagnostic table. Refer to Crankshaft Position System Variation Learn.

Start the engine.
Allow the engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records.
Monitor all of the Misfire counters with the scan tool.
Are any of the Misfire current counters incrementing?
--
Go to Step 3
Go to Diagnostic Aids

3
Are any other DTCs set?
--
Go to Diagnostic Trouble Code (DTC) List
Go to Step 4

4
Can any abnormal engine noise be heard?
--
Go to Symptoms - Engine Mechanical
Go to Step 5

5
Does the scan tool indicate that the HO2S bank 1 sensor 1 or HO2S bank 2 sensor 1 voltage parameters are below the specified value?
200 mV
Go to DTC P0131 or P0151
Go to Step 6

6
Does the scan tool indicate that the HO2S bank 1 sensor 1 or HO2S bank 2 sensor 1 voltage parameters are fixed above the specified value?
900 mV
Go to DTC P0132 or P0152
Go to Step 7

7
Inspect the following components:

   •  The vacuum hoses and seals for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram.

   •  The throttle body and intake manifold for vacuum leaks

   •  The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.

   •  The PCM grounds for corrosion and loose connections--Refer to Ground Distribution Schematics in Wiring Systems.

   •  The exhaust system for restrictions--Refer to Restricted Exhaust in Engine Exhaust.

   •  The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

Did you find and correct the condition?
--
Go to Step 20
Go to Step 8

8
Important: An erratic or inconsistent spark is considered a no spark.

Turn OFF the ignition.
Disconnect the spark plug wire from the spark plug that corresponds to the Misfire Current counters that were incrementing. Refer to Spark Plug Wire Replacement.
Install the J 26792 Spark Tester.
Start the engine.
Does the spark jump the tester gap?
--
Go to Step 10
Go to Step 9

9
Remove the spark plug wire for the affected cylinders. Refer to Spark Plug Wire Replacement.
Inspect the spark plug wire. Refer to Spark Plug Wire Inspection.
Measure the resistance of the spark plug wire with a DMM.
Is the spark plug wire resistance less than the specified value?
700 Ω
Go to Electronic Ignition System Diagnosis
Go to Step 19

10
Remove the spark plugs from the cylinders that indicated a misfire. Refer to Spark Plug Replacement.
Inspect the spark plugs. Refer to Spark Plug Inspection.
Does the spark plug appear to be OK?
--
Go to Step 11
Go to Step 12

11
Exchange the suspected spark plug with another cylinder that is operating properly. Refer to Spark Plug Replacement.
Operate the vehicle under the same conditions that the misfire occurred.
Did the misfire move with the spark plug?
--
Go to Step 18
Go to Step 15

12
Is the spark plug oil or coolant fouled?
--
Go to Symptoms - Engine Mechanical
Go to Step 13

13
Is the spark plug gas fouled?
--
Go to Step 16
Go to Step 14

14
Did the spark plug show any signs of being cracked, worn, or improperly gapped?
--
Go to Step 17
Go to Step 15

15
Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test.

Did you find and correct the condition?
--
Go to Step 20
Go to Symptoms - Engine Mechanical

16
Perform the fuel system diagnosis. Refer to Fuel System Diagnosis.

Did you find and correct the condition?
--
Go to Step 20
Go to Symptoms - Engine Mechanical

17
Replace or gap the spark plug. Refer to Spark Plug Replacement.

Did you complete the action?
--
Go to Step 20
--

18
Replace the faulty spark plug. Refer to Spark Plug Replacement.

Did you complete the replacement?
--
Go to Step 20
--

19
Replace the faulty spark plug wires. Refer to Spark Plug Wire Replacement.

Did you complete the replacement?
--
Go to Step 20
--

20
Was the customer concern the MIL flashing?
--
Go to Step 21
Go to Step 22

21
Operate the vehicle at the specified value for 4 minutes.
Operate the vehicle within the Conditions for Running the DTC P0420 or P0430 as specified in the supporting text. Refer to DTC P0420 or P0430.
Does the DTC run and pass?
2500 RPM
Go to Step 22
Go to DTC P0420 or P0430

22
Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 2
Go to Step 23

23
Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List
System OK

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