06/06/2026 6:12 am
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Automotive Q&A
Cold-start P0300 is concentrated on cylinder 6 for the first 20 seconds
Review the evidence, add a focused answer, and distinguish confirmed measurements from assumptions.
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A 2017 Silverado 5.3 has a rough start only after sitting overnight. The scan tool records P0300, but the live counters show nearly all events on cylinder 6 and they stop within 20 seconds. Compression is even, ignition parts have already been swapped, and the cooling system holds pressure. The cylinder 6 plug is darker and smells of fuel after the overnight soak.
Would fuel-pressure decay and injector balance be the next useful branch, or could the short duration still point to a mechanical issue?
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07/06/2026 9:32 am
The cylinder-specific wet plug makes an injector leak plausible, but prove it without losing the cold-start condition. Record rail pressure after shutdown, compare injector balance using the service procedure, and inspect cylinder 6 with a borescope before the first start. A pressure drop alone is not enough because the pump check valve and another injector share the boundary.
If the balance test for cylinder 6 differs and the borescope shows fresh fuel while the other cylinders are dry, the injector is supported. Keep compression and coolant evidence in the record so the conclusion does not rest only on the misfire counter.
Topic starter
08/06/2026 10:35 am
Rail pressure fell from 58 psi to 19 psi overnight. Cylinder 6 showed a wet piston crown before cranking and its balance drop was larger than the other seven injectors. After replacing that injector and its seals, overnight pressure stayed above 50 psi and the cold-start counter remained at zero.