P0300 Code 2001 Silverado 4.8L Cat Conv.?

Discussion in 'Maintenance' started by Jim Jersey, Dec 12, 2016.

  1. Jim Jersey

    Jim Jersey New Member

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    Hi folks. 2001 Silverado LT Ext Cab 4.8L 236K miles all original. Been getting P0300 multiple random misfires. Happens after highway driving about 45-50 minutes and appears when I decelerate like down a hill. (almost like truck breaking action), MIL flashes, code set. Looked at possibilities; Cat. Conv., intake leaks. Believe all ignition good, fuel pump replaced so leaning towards CC and intake. I also get like creaking/ knocking sounds from exhaust (can't tell, maybe Cat. Conv.) , after parked and cooling, occasional once when I first start engine. Any thoughts on how to check Cat Conv., other isolation, anyone seen this before.Thanks.
     
  2. John Clark

    John Clark GMFS Member

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    When the vehicle idles does it idle smooth or is it idling rough? You need to find which cylinder or cylinders are misfiring and then figure out what is causing it. It could be anything from a spark plug or plugs, plug wire(s), coil pack, injector or even a compression issue.
     
  3. Jim Jersey

    Jim Jersey New Member

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    Thanks. The idle is OK and for the most part it runs fine. The P0300 is random misfire. In theory if it were one or two cylinders OBD2 should call out the specific one; P0301, 02, etc. I can check all them individually but what is telling is that after a time, when I decelerate on highway it starts which leads me to believe it is more vacuum or exhaust related. I have tried to capture the frames on OBD2 while driving and it occurs but hard to do and anticipate at 65-70 mph.
     
  4. John Clark

    John Clark GMFS Member

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    Keep in mind that the misfire counters and the specific misfire cylinder codes (P0301-306) are not always accurate. The way it works is that the PCM detects a sudden decrease in engine RPM and the compares it to where the crankshaft is and that's how P0301-306 are set. They have gotten better over time but in the early 2000's they weren't always accurate.

    The reason I asked if it missed at idle was that you can unplug each cylinder one by one and see if there is any RPM change when you unplug it. If so, that means that cylinder is contributing. If no change then you've found your problem cylinder.

    You might be able to see an issue with fuel trims if it's a vacuum leak. What are your fuel trims looking like at idle and at around 3000 RPM?
     
  5. Jim Jersey

    Jim Jersey New Member

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    Thanks John, Ill check them when I get a chance
     

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