07 suburban p0300 can’t figure out

[nvious62]

So I was driving my 07 5.3 flex fuel suburban home with the family when started running really rough. Pulled over and had p0300 code. Motor shakes bad sound like has a knock and it’s really hard to start. Had it towed home and started checking things. Fuel pressure is 55 at the fuel rail. The plugs and wires were replaced about 2000 miles ago. Changed the map sensor checked ground for coil packs. I put my scanner on there and it is random misfires on 1,2 and 4. With 4 having the most misfires. Would greatly appreciate any help with this

 

[89Suburban]

Is your AFM active?

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

 

[Fless]

What do the short-term fuel trims look like?

How much ethanol is in the gas you run (10%, 15%, or a version of E85)?

What does your scanner say that the fuel alcohol content is? Does that seem to match what fuel you're using? If the calculated alcohol content is much higher than the actual content, the computer will mistakenly add fuel, making the mixture way too rich. If that's the case and your scanner can reset the alcohol content, do it.

 

[nvious62]

Here is what I was able to pull with the cheap scanner I have I got better one but haven’t had time to hook it up. I’m running 87 with 10% ethanol.

 

[Fless]

If you've not run any E85 lately, then the computer is mis-calculating the alcohol content; it should be 10% or less based on using E10. Need to get a scanner on it that can reset the alcohol percentage. That'll be a start to see if it helps the misfires.

 

[nvious62]

No afm is disabled thru a tuner.

 

[nvious62]

Ok will try to see if new one I have can do it or if I can find scanner.

 

[nvious62]

Ty everyone for the help this has been driving me nuts. The only thing I forgot was I just filled up with gas the day that it happened from an Exxon station.

 

[Fless]

If you suspect tainted fuel, take a sample (like a cup or two) from the fuel rail and let it sit a while in a jar. You may be able to visibly see contaminants. If it separates and has water in it, the water will be on the bottom.