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my engine randomly misfires only at highway speeds code p300.when at city speeds the engine has smooth idle no flashing cel no bucking or surging even at highway speeds no bucking or surging.the cap and rotor was replace with new platinum plugs ac delco and wires the engine is also running lean p0171-p0174 i believe the p300 is the underlying cause of the lean codes.as far as the lean code there is NO vaccum lines are leaking no hissing,the maf sensor replace,the head gasket is new NO leaks at all,pcv valve replaced.air intake is good no cracks at all clamps are tight,fuel injectors are brand new (these were replaced with the head gasket as well),bank one oxygen sensor replaced,fuel filter was replaced last spring 2015,it also passed a fuel balance test as well. IM REALLY STARTING TO THINK THAT THE LEAN CODES P0171&P0174 AND THE P300 IS CAUSING ONE ANOTHER TO SET CODES.

Vehicle: 2004 Chevrolet Trailblazer 4.3L Vortec

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P0300 Random misfire

Right at the ignition coil if you have a spray bottle make a solution of water and salt and spray at the ignition coil, that area there tends to leak secondary voltage at time,and goes unnoticed as you spray the suspected area spray and listen for any secondary ignition leak

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