2003 4.8 Z71 P0300 DTC - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-05-2016, 04:19 PM   #1
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Default 2003 4.8 Z71 P0300 DTC

I have a 2003 silverado. Starts up fine and idles fine when cold. Once it warms up, it starts idling rough. I've tried unplugging COP'* one at a time while rough idling and found that I can unplug all 4 coils (1,3,5,7) at the same time and idle will not change. If i unplug only one coil (2,4,6, or 8 ) it significantly affects the engine. All 8 coils have spark even during rough idle. When it rough idles, i get a raw fuel smell from the tail pipe.

If i get on an open highway/freeway, it will not act up so long as I'm moving. If i, for example, hit stop-n-go traffic, it starts sputtering, sometimes making a "clunk" sound when I take off.. almost sounds like the drive shaft fell off. But, once I get going, after a little hesitating, it just takes off and the misfire stops. Until I again come to a stop.
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Old 02-06-2016, 03:12 PM   #2
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It may be best to have a shop with an advanced scanner take a look at this.
Normally start by doing a full tune up, installing new OEM spark plugs.
A 13 year old vehicle may need all the coil packs replaced.
A Snap on scanner with a low amp voltage probe, under current ramp test/labscope can view the coil pack individually firing.
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Old 02-06-2016, 09:58 PM   #3
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Would it be swap the coil wire harnesses and see if the other cylinder bank refuses to fire.
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