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04 Sierra P0300

SargeSlaughter
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SargeSlaughter Enthusiast

SargeSlaughter

Posted
Posted

Does anyone have any advice where to start looking for a P0300?

 

Truck has LM7 with 188k on the clock. Runs fine until it starts to warm up or going up a grade and starts missing and spitting. Things I've replaced over the last year,  throttle body, maf, map, cats and all o2 sensors, plugs and wires. Replaced coils 2 years ago. Fuel pump was just replaced with a delphi, check engine light would go on and it would spit and sputter, wouldn't start one day so changed the pump and it started but still spits and sputters when its under a heavy load.

 

Haven't changed the cam or crank position sensor, pedal assembly, intake gaskets or injectors.

 

Its giving me a flashing p0300 when going uphill but them it turns off when I'm back on level ground and no extra stress is on the motor. Passed smog like a champ 4 months ago. No clue what could be going on.

CamGTP Senior Enthusiast

CamGTP

Posted
Posted

Get a live data scan tool on the truck to see exactly what cylinder(s) are misfiring. That will make it far easier to track down what to look at.

 

It could be just one cylinder or an entire bank of cylinders but it's hard to say right now.

Brian715 Enthusiast

Brian715

Posted
Posted
On 9/20/2020 at 7:30 PM, CamGTP said:

Get a live data scan tool on the truck to see exactly what cylinder(s) are misfiring. That will make it far easier to track down what to look at.

 

It could be just one cylinder or an entire bank of cylinders but it's hard to say right now.

+1 

SargeSlaughter Enthusiast

SargeSlaughter

Posted
  • Author
Posted

Misfire counter shows its all over. Sometimes it doesn't start and takes 2 or 3 tries to get it going. Also I noticed the Delphi pump I pump in it has a working pressure of 54psi and the spec on these trucks is 55 to 62. Could that be a possible culprit? I checked pressure while the line was primed and am only get about 50psi. Maybe a bad pump?

CamGTP Senior Enthusiast

CamGTP

Posted
Posted

The pressure is fine, it's not low enough to be an issue. You'd have issues if the pump could barely even do 45-50psi while running and under load.

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