excesive fuel consumption

GMC SONOMA
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CHEVY169
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I have a 2003 zr2 Im getting a p300 random multiple cyl missfire code and when I got the truck it was getting 21mpg and now it gets like 15 mpg. the truck runs great and has always ran great no change did full tune up when light came on also ran injector cleener through it
thanks rob
Sep 27, 2007 at 8:40 PM
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RASMATAZ
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Tune-up parts are they OEM parts? P0300 is normally caused by vacuum leaks and bad gas.

Might want to check the airfilter and fuel filter conditions to complete your tune-up.

loss of compression, and anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure.
Sep 28, 2007 at 2:22 AM
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CHEVY169
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Replaced all filters, wires plugs cap and rotor good pressure no vacume leaks. the dealer has it now and they ran injector service on it and gave me a 6 page instruction on reseting the computer. the light still comes on. is it possable that the internals of the intake leak. they say that the valves have varnish on them causing them to stick. but with 17k miles since the code came up I have a hard time beleving that.
Jan 14, 2008 at 11:48 AM
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RASMATAZ
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Could be valve problems-if they did a compression check
Jan 14, 2008 at 1:18 PM