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My 04 sierra 6.0 has an intermitant missfire code at speeds above 60 MPH. The check engine light goes out after driving at lower speeds. I did replace the plug wires and plugs.
Vehicle: 2004 GMC sierra Denali 6.0

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Misfire condition above 60 MPH, the only cause for above 60 MPH  misfire condition is a partial plugged exhaust system. These engine are sensitive, if allowed to remain too long what happens is it develops a crack between the intake valve and exhaust valve. Then you would end up with a misfire at idle.

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