05 GMC SIERRA 5.3 V8 Truck with code PO300

[lisa Williams]

I have 05 GMC SIERRA 5.3 V8 4 wheel drive truck. Great truck. Had a little oil leak so decided to get valve cover gaskets replaced when I got my oil and filter done. Since then it sputters/hesitates and throws PO300 CODE. Changed 02 sensor and plugs. Still doing it and throwing the code. I don't have money to keep buying to not fix.  It ran brand new til had valve cover gaskets done

[CamGTP]

When the valve cover gasket are replace, it requires plug wires and coil packs to be removed to get the covers off. It's very possible that some of the plug wires got damaged or maybe some of the wires going to the coil packs got damaged somehow.

 

P0300 just means multiple cylinder misfires. So more than 1 cylinder is having an issue. It's always best to use a scan tool so that you watch and see what cylinders are misfiring. Way too often people just start throwing parts and money at the vehicle and it only costs them more time and pain to the wallet. You'd save yourself money if you do the proper steps to figure out why it's throwing that code.

[lisa Williams]

My 05 GMC SIERRA has recently started running crappy. One it hits like 55mph it's ok. Had guy change my calve cover gaskets cause they'd been leaking for awhile. Mind u it ran fine til then. Grommets were not replaced. Drove bout 20 miles and check in goes to flashing and it starts misfiring and runs ragged til u hit bout 50 or 55. Checked codes it's a po300 code. So he suggested new spark plugs. Then it threw code for misfire on 4 so we changed that wire to plug plus put an 02 sensor. Back to code PO300 and it runs just as crappy as b4. We'll this mechanic says he feels full injector needs to be changed to #6 cause he took wires off and zero happened at number 6. So changed it. Still no better.  I'm a woman and  I don't have it to keep buying and it's still no better. Any suggestions on what I can do to figure it  out? I'm bout ready to just be done with it. Crazy thing is....it ran perfect til valve cover gaskets replaced. Any suggestions....I'm desperate 

[davester]

I would start with the ignition side, doing the valve covers involves at least unplugging, if not unbolting the coils, so possibly the wiring hasn't been fully plugged in or damaged when that work was done.

 

You might use a bidirectional code reader, that can display live data for misfires, to see if they are spread across all cylinders, just one bank, or just one or two cylinders.

 

And it sounds like you have found a mechanic that throws parts at it, vs doing diagnosis.  You might find one that does diagnosis.