Two weeks ago I got the dreaded p0300 engine code. Since then I've replaced spark plugs, wires, checked all coils and replaced one, installed a new fuel pump, new map sensor and I have a new maf. Yesterday I had it towed to a dealer for a diagnosis, they said all was fine with fuel, spark, air and advised that #1 & #3 had bad valve springs. I checked them today, #1 had a some play (spun easy) in the exhaust valve spring after bumping the engine over to get tension off. As for #3 there was very little play in it. I still find it hard to believe that valve springs can cause the motor to misfire.
03 Sierra has p0300 code
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Po300 is usually injector related. Go thru while its running and pull plug wires, whichever sylinder DOESNT change the way it runs, theres your problem.
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