Question

I have a rough idle on 2004 Chevrolet Silverado 2500HD 6.0L, but smooth when running

Answer 1

Rough idle usually will set a misfire code condition. If there is no CK ENG LT or any pending code for the rough idle, check the sparkplug for proper torque

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Sparkplugs should be checked at check to see if they are loose, you do not necessarily have to remove the sparkplug wire, when the engine is at idle lightly tug on the sparkplug wire if it is not torque down properly you would feel it moving during the idle