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P0300 & P0455 MISSFIRE & EVAP HELP!

P0300 & P0455 MISSFIRE & EVAP HELP!

Postby mxz600rr [OP] » May 18 2014, 7:18pm

Hey guys

I have a 2004 Chevy Silverado 1500 Z71 4x4 and I've been having some issues and i need help!

About a year after i got my truck i put some sea foam in the tank and it did too good of a job. The engine started misfiring and it turned out that all 8 injectors got clogged. so i replaced them and cleaned the fuel rails, it fixed the issue but now on highway drives if I'm doing 100km/h for a while or if i go around 120km/h then let off the throttle the check engine light will start flashing and throws a P0300 code Random engine misfire. I notice no lose in power or any changes it will just throw the code. I don't know whats causing it all plugs and wires were changed the only other thing i can think of is maybe somethings wrong with the fuel pump assembly? Please Help!

Also over the past few months it will randomly throw a p0455 code Large EVAP leak, and on my dash it says tighten gas cap, i replaced the gas cap but it still does it. i notice it happens when i have in-between 1/2 and 1/4 tank of gas. HELP!
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Re: P0300 & P0455 MISSFIRE & EVAP HELP!

Postby mxz600rr [OP] » May 26 2014, 1:44am

EVAP code comes up right away as well when i fill the tank to full PLEASE HELP!
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Re: P0300 & P0455 MISSFIRE & EVAP HELP!

Postby [email protected] » May 29 2014, 4:24pm

Pretty sure that truck will have an ERG system, but I could be wrong. I know some of those components can be pcv/plastic and crack, I'd try getting that fixed as it may be the culprit for the random mis-fire as well.
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Re: P0300 & P0455 MISSFIRE & EVAP HELP!

Postby Bigsmith1993 » May 29 2014, 4:54pm

The p0300 code can be cause by something as simple as a vacuum leak, a vacuum leak can also cause the evap code as well, I would check all the evap hoses and all tank connections to make sure it's sealing off, if it's not sealing off properly, the tank won't pressurize or vacuum down causing it to throw the code.
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