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Old 07-11-2017, 01:51 PM   #1
bhayes1
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P0300 code

During a family road trip my check engine light came on so I ran the code and it came back as P0300 random misfire. I filled up at a shady place so I figured maybe I got some water in my tank. Ran a can of sea foam through it and it cleared it but came back shortly after.

Truck couldn't possibly be running any better. No missing, sputtering etc.

From what I read this code is hard to diagnose because it doesn't give you a specific cylinder as a starting point. I hate to just start throwing parts at a truck that runs so good. Anyone have any tips....
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Old 07-11-2017, 05:15 PM   #2
Hidesertchevy
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Re: P0300 code

Plugs and wires is what I would start with
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Old 07-11-2017, 08:58 PM   #3
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Re: P0300 code

That's what I'm leaning toward.
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Old 07-12-2017, 07:15 AM   #4
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Re: P0300 code

How many miles are on your truck? And does the engine actually feel like it is misfiring? Or does it still run smooth? The 99-06 trucks have been known to throw the po300 after 120,000+ miles because the timing components wear, but the crankshaft position sensor still reads to the factory specs. This causes it to think there is a misfire under long driving times, or under heavy load. I went through this with my Silverado. The engine runs amazing at 170k miles with no sign of misfire. I'm not saying that this is definitely your issue, plugs and wires wouldn't hurt. But if it is the same issue, you'll have to go in with a scan tool and do a re-learn on your crankshaft position sensor. Hope this helps.
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Old 07-12-2017, 10:18 AM   #5
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Re: P0300 code

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Originally Posted by Country Suburban 4x4 View Post
How many miles are on your truck? And does the engine actually feel like it is misfiring? Or does it still run smooth? The 99-06 trucks have been known to throw the po300 after 120,000+ miles because the timing components wear, but the crankshaft position sensor still reads to the factory specs. This causes it to think there is a misfire under long driving times, or under heavy load. I went through this with my Silverado. The engine runs amazing at 170k miles with no sign of misfire. I'm not saying that this is definitely your issue, plugs and wires wouldn't hurt. But if it is the same issue, you'll have to go in with a scan tool and do a re-learn on your crankshaft position sensor. Hope this helps.
Never heard of this, but this sounds exactly like what I have going on. Truck is a 2011 with a 5.3

The light just went back out since I got back home yesterday and did some in-town driving. I'll have to do a little research on the re-learn and see what I come up with.
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Old 07-12-2017, 10:28 AM   #6
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Re: P0300 code

I just bought this cool little gadget made by Foseal on Amazon. It plugs into your obd2 plug and you use your phone to read the codes and clear them as well. A little research on Youtube and I found the leaking vacuum hose.
Not sure about your issue but I like this little thing and it cost about $17.99.
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Old 07-12-2017, 10:41 AM   #7
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Re: P0300 code

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Originally Posted by bhayes1 View Post
Never heard of this, but this sounds exactly like what I have going on. Truck is a 2011 with a 5.3

The light just went back out since I got back home yesterday and did some in-town driving. I'll have to do a little research on the re-learn and see what I come up with.
Another thing I've run into with that po300 was the 15A fuse that powered my two upstream O2 sensors, it was blown and once I replaced it, my O2 code and misfire code was gone and hasn't come back. It's amazing what can cause issues in these computerized and electronic components.
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