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2004 GMC Sierra 2500 HD Question: Code P0300 Missfires at idle ONLY

 

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Kimhoug60, 6.0L V8, Goleta, CA, October 19, 2010, 17:17
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New spark plugs uridium spark plugs. New wires. Changed o2 sensors. Still has the same code. With scan tool check missfires found in all cylinders at idle ONLY. Just off idle missfires stop, 100+ RPMS off idle. Vehicle has a new throttle body installed and a new charcole canister all from dealer. New comp cam and lifters installed.

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  • Answer #1

    patrick mannion from Greg Solow's Engine Room, October 19, 2010, 20:06
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    I wonder could you have a vacuum leak? Many automotive repair shops have a machine that generates an inert non flammable low pressure smoke that looks similar to cigar smoke. The intake manifold is sealed off, and smoke is put into the engines intake manifold. The smoke then fills the engines crankcase and if there are any vacuum leaks from gaskets, hoses, or housing smoke emits from the source of the leak. Some mechanics will use propane or aerosol carburetor cleaner, with the engine running they will spray the cleaner or propane around the intake manifold and vacuum hoses. A leak will cause a "lean mixture" condition, when the flammable fuel (carburetor cleaner or propane) comes in contact with the source of the vacuum leak the engine idle speed will raise as the engine has received "fuel". this method of finding a vacuum leak works but is not recommended as it can easily cause a fire. Looking at live sensor data on a professional grade scan tool is a great help. What are the fuel trim figures like short term for both banks. If you end up with your truck at a repair shop and they are still stumped, there is a professional service called Identafix. http://www.identifix.com/info/about_us.htm They only deal with professional repair shops. They are in the Minnesota 651-633-8007 begin_of_the_skype_highlighting 651-633-8007 end_of_the_skype_highlighting. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people.If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base.

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  • Answer #2

    October 20, 2010, 07:29
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    I am concerned with the cam. what is the valve overlap degree?? with a comp cam, the idle is most vulnerable to erractic operation. these are made to idle higher than spec to generate enough vaccuum to make everything work. there are software available to change timing and idle speeds from the manufacturer of the cam. I would check into that first.

    Roy

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