Chasing a P0300

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Cornelius

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Long time lurker, seldom poster here.
2003 Yukon, 5.3l, 4wd, 215k
A few months ago it popped a p0300
It runs fine, pleanty of power, good throttle response, no other codes except for the p0300
I'm starting to run out of inexpensive parts to throw at it, and so far what I've done is what is expected for either maintenance or age/mileage:
Plugs and wires
PCV valve
Intake manifiold gaskets
Rear O2 sensors (no voltage detected but no codes either)
Checked vacuum lines, good

I could certainly keep going with parts, like MAF sensor, knock sensors, cats, FPR, Injectors, etc but I'm not getting ANY OTHER CODES!

Any guidance on where to start from here would be appreciated. I hope to find an app that will give misfire counts for each cylinder, then I can start swapping around coils.

Will knock sensors (or anything else) cause a p0300 without lighting a CEL with it's own code?
 

exp500

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Even the Torque App will show enough live Data to figure out Where usually. Make sure you have the PIDS for GM installed. Kinda tedious to set up first time because you are adding so many monitors. Or start with a dozen and add more later.
 

BG1988

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Long time lurker, seldom poster here.
2003 Yukon, 5.3l, 4wd, 215k
A few months ago it popped a p0300
It runs fine, pleanty of power, good throttle response, no other codes except for the p0300
I'm starting to run out of inexpensive parts to throw at it, and so far what I've done is what is expected for either maintenance or age/mileage:
Plugs and wires
PCV valve
Intake manifiold gaskets
Rear O2 sensors (no voltage detected but no codes either)
Checked vacuum lines, good

I could certainly keep going with parts, like MAF sensor, knock sensors, cats, FPR, Injectors, etc but I'm not getting ANY OTHER CODES!

Any guidance on where to start from here would be appreciated. I hope to find an app that will give misfire counts for each cylinder, then I can start swapping around coils.

Will knock sensors (or anything else) cause a p0300 without lighting a CEL with it's own code?
i'd check the throttle body have you tried cleaning that?
Air filter?
also check the hose that pops into the intake tube
 
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Cornelius

Cornelius

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i'd check the throttle body have you tried cleaning that?
Air filter?
also check the hose that pops into the intake tube

air filter is clean
Throttle body I sprayed when I did the intake gaskets but yeah that would be a free one to eliminate
Which intake tube?
 

BG1988

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air filter is clean
Throttle body I sprayed when I did the intake gaskets but yeah that would be a free one to eliminate
Which intake tube?
ah might be different on the nbs intake tube

but there could be a crack in it causing a leak
 
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Cornelius

Cornelius

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Throttle body is now clean
The vacuum line on pass side that appears to be a breather hose that goes from valve cover to intake was good but each end was a tad stretched so I hose clamped them on tight

does anyone know of a iOS app that supports misfire counts on each cylinder? I use a wifi ELM27 adapter
 
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Cornelius

Cornelius

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Still trying to figure this out. I don't drive it that much (company car) and life has taken priority but now I've gotten back to it.....

just to eliminate a cheap part i replaced the fuel pressure regulator
bought a fuel pressure tester, pump pressure is 50 when running so that seems good. holds 40 after an hour. not sure how much pressure is supposed to hold but i suspect it bleeds off back toward the tank.
 
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Cornelius

Cornelius

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Now I’m looking at the maf sensor live data but unsure of what is normal for how it fluctuates when RPMs change. The numbers are right from idle to WOT but I don’t know if the line on the graph is supposed to be a smooth slope, or jagged line
 

ivin74

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Now I’m looking at the maf sensor live data but unsure of what is normal for how it fluctuates when RPMs change. The numbers are right from idle to WOT but I don’t know if the line on the graph is supposed to be a smooth slope, or jagged line


You need to find someone with a tech 2 and have them look at the live data. I'm willing to bet you have a bad coil.
 
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Cornelius

Cornelius

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You need to find someone with a tech 2 and have them look at the live data. I'm willing to bet you have a bad coil.
I can see live data through OBD adapter and app. No codes for individual coils and when I unplug them one at a time I can hear the cylinder go down. Do better scanners read better? I know they can dive deeper but I was hoping that I can see enough through a phone app to solve this.
 

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