......There are 4 major causes of misfires.
1. Spark plug and boot related.
2. Coil related.
3. Fuel related.
4. Cylinder mechanical issues.
......How is misfire detected?
A. The system measures crank rotation time against an average time table for all cylinders and can self adjust for reasonable engine wear over life.
B. When a cylinder is slow on rotation time for any reason, it is a logical misfire in the digital system the engine runs on.
C. Secondary misfires such as above are able to trigger fault codes in the 301 to 310 code group for 6, 8, and 10 cylinder engines found in these trucks.
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...... A second code group for misfires are the 351 to 360.
...These codes are NOT based on rotation time of the Crank but an electrical circuit performance check each time the coil is addressed in the firing order.
...How this is done is by the computer monitoring the 'current' waveform profile for known response from the coil's electrical action when pulsed on and off. The spark occurs when the operating ground from the computer is opened not when it is applied. The reason is the coil need magnetic saturation time to develop the spark before the time the ground is opened.
...Obviously if the waveform is absent the code is set for that cylinder. If the waveform is outside normal parameters it is still considered an electrical fault.
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......Special misfires that don't set codes.
...During specific driving conditions in overdrive at light throttle, the Exhaust Gas Recirculation system is called into operation. This greatly 'Leans' the Air to Fuel ratios in the cylinders not by lack of air but from contamination by exhaust gases and a reduction of fuel injected under those conditions.
....Misfires under these conditions are very often caused by one or more faulty coils. The reason is the coil/s has a fault that limits it's maximum spark voltage. Under these conditions of very lean mixtures, the spark voltage required increases greatly. If a coil cannot spark with enough high voltage a misfire results. Generally the driving condition is best described as a stumble that goes away with down shifting until the next time the same driving conditions are present.
....What is the coil fault? Normally a complete failure sets a code 30x or 35x as describes above. In this special case, the coil has developed 'shorted' turns in it's winding. Because it is not an outright hard failure and still generates spark at other times, no code is set because the program cancels the misfire record for normal codes 30x after the cylinder recovers normal operation in a non EGR driving mode.
....How is the faulty coil detected?
Good you ask!!! In the program at another software location, a monitor records the misfires for each cylinder. In order to look at these records a SCANNER is used to access Mode 6, test 53. At that location you will find the records of each cylinder. The cylinder/s with the high counts out of limits will usually be the ones that need the coil replaced due to shorted turns as indicated above.
....This table record has no ability to set code so it needs to be looked with a good diagnostic Scanner. A Reader is not a Scanner.
.... Often looking at these tables can suggest a boot with pin holes, carbon tracking etc. and may even indicate a pending issue with that cylinder's ignition before they show up as a drivability issue.
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.....Other misfires.
A. Cylinder issues with blown head gasket.
B. Coolant getting into a cylinder from gasket failures.
C. Heater hose leakage onto coil areas.
D. Coil plug seating or harness issue.
E. Fuel injector operation same as coil failures etc.
F. Low compression for any reason affects cylinder rotation time.
G. Sometimes the Crank sensor plug may not be seated and subject to vibration movements.
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$$$$$$$ Finally, do not ignore the special CEL flashing indication as it can be extra costly. The reason the CEL is flashed by the computer is when the program decides that due to a misfiring cylinder, Raw fuel is passing through the cylinder to the Cat converter. At this point the unburnt fuel can ignite in the CATS causing very high temperatures and melting the substrates. This can further cause blocking the exhaust, a reduction in engine power over just one cylinder and huge costs to replace the converter.

****Bottom line is to diagnose from a position of understanding and not throw parts at the problem hoping for a 'jello to stick to the wall' solution, spend money, time, fail and the frustration it may cause.
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Use logic, test, observe, fit the issue to the possibility, then act.
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Good luck.

 

Ill be saving this one ,, a good reminder,,thanks for posting