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I have an 02 Chevy Silverado 1500 4x4 with the 5.3 getting the po300 random misfire code. It idles great and drives fine if I baby it but as soon as it gets any load on it it starts to miss. I have done new plugs/wires, knock sensors, cam position sensor, new gaskets from the heads up, cleaned all of the injectors and MAF sensor, new fuel filer, all new coil packs and had the cats checked and they are not clogged. Been dumping time and money into it with no luck so far. Any ideas?

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    Check for vacuum leaks on hoses connected to the intake manifold. – Moab Jan 16 at 2:39
  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Jan 16 at 11:45
  • Agree with @Moab here, especially the large one connected to the throttle body (after the MAF). You're looking for unmetered air getting into the system. If you have access to a code reader which will show live data, you could check the fuel trims. More than likely you are experiencing a lean misfire. If the computer isn't able to counteract the lean condition, you should see it dumping a lot of fuel through the fuel trims (long term/short term). If these are maxed out, you've most likely got a large leak in your intake system. – Pᴀᴜʟsᴛᴇʀ2 Jan 16 at 15:03

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