GM 6.0 Misfire Under Load – Secondary Ignition Failure (Silverado/Sierra)

Diagnosing a severe shudder or misfire on a GM 6.0L Vortec V8 (LQ4/LQ9) that only occurs under heavy load, such as towing or hard acceleration. This issue is the definitive sign of a secondary ignition failure, not a dirty Mass Air Flow (MAF) sensor. Under high cylinder pressure, the electrical resistance across the spark plug gap increases dramatically. If an ignition coil is weak, a spark plug wire's insulation is cracked, or a plug gap has eroded too wide, the 40,000-volt spark will \blow out\ and arc to a ground instead of firing the plug. This creates a dead miss on one cylinder. The correct diagnostic procedure involves using a bidirectional OBD2 scanner to monitor live misfire counters (Mode $06 data) to identify the failing cylinder. Swapping the suspect coil and wire to another cylinder will confirm if the misfire follows the hardware, pinpointing the exact cause of the catalyst-damaging event.,
tags:
6.0 misfire under load,
lq4 misfire,
lq9 misfire,
silverado 2500hd misfire,
secondary ignition failure,
spark blowout,
vortec 6.0 problems,
flashing check engine light,
ignition coil test,
mode 06 misfire data

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