I tried replacing all the spark plugs, ignition coils, crankshaft sensor, camshaft sensors, replaced fuel injectors, replaced plenium manifold gaskets, checked compression in all cylinders, checked fuel pressure, checked for intake leaks, Checked for spark from ignition coils, checked timing chain, replaced valve covers due to oil leaking in spark plug chamber, replaced lower oxygen sensors, replaced drivers side cat.converter
I have a 2004 infiniti g35 coupe and was getting the exact code along with misfiring in one or more cylinders....etc on my OBD II scanner. Along with a rough idle and it would sound like it was going to stall but didnt. I replaced both VVT solenoids and it runs great again. I tried cleaning the screens in the solenoid first. I read online from a few people that thats all they needed to do. But it didn't work for me. Good luck!!
0300 code can be caused by defective or clogged VCV valve module - "Vapor Canister Valve" is their name for what is an EGR valve. If you smell tons of gas when starting it cold, this is your most cost effective culprit. Part is about $20 to $90 USD.
Bad gas with too much ethanol or water can cause a lean issue and P300 codes .
SOURCE: 2002 Chevy Silverado po300 po449 codes
Trouble Code: P0300 Multiple Cylinder Misfire Detected Possible Causes: EVAP Vent Solenoid Control Circuit Malfunction Possible Causes:
Base engine mechanical fault that affects one or more cylindersFuel delivery component fault that affects more than 1 cylinderEVAP system problem or the EVAP canister is fuel saturatedEGR valve is stuck open or PCV system has a vacuum leakIgnition system fault (a coil) that affects more than one cylinderTSB 03-06-04-030 contains a repair procedure for this codeTSB 03-06-04-041 contains a repair procedure for this codeTrouble Code: P0449
Vent solenoid control circuit is open or shorted to groundVent solenoid control circuit is shorted to system power (B+)Vent solenoid power circuit is open (test the ENG1 fuse)Vent solenoid is damaged or has failedPCM has failed
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