03 2500hd 6.0 misfire issue

[ShiftyEskimo]

Misfire with flashing check engine light under load at around 2k rpm. Fuel pressures a tad low at 44-55 after a new fuel pump, replaced the intake manifold gasket today ran great around the block once and back to flashing light. Codes are a mix of p0300 and p0307 switching intermittently between the two and a p0171. Any other ideas before I swap injectors?

[Jsdirt]

I'd wager the new fuel pump isn't moving the required amount of fuel. That could be either due to a plugged or defective fuel filter, or a defective pump.

 

Also, low output can be due to wiring issues. If you're having the exact same issue after replacing the pump, then it's time to check wiring and amp draw. Fuel pump wiring should power a halogen bulb nice and bright - if it doesn't, that needs to be investigated. Powers AND grounds - both are equally important as the load will not work with either of them bad, let alone both.

 

Always check areas of heat and vibration, and ANY connections. Those are the usual suspects.

[asilverblazer]

On 1/15/2026 at 1:19 PM, ShiftyEskimo said:

Misfire with flashing check engine light under load at around 2k rpm. Fuel pressures a tad low at 44-55 after a new fuel pump, replaced the intake manifold gasket today ran great around the block once and back to flashing light. Codes are a mix of p0300 and p0307 switching intermittently between the two and a p0171. Any other ideas before I swap injectors?

I don't recall if the P0171 is bank specific - if it is, try swapping injector 7 with one from another bank and see if the lean condition and miss fire follows injector 7 to the other bank/cylinder. 

 

I'd focus on injectors, maybe clean mass air flow sensor to make sure its not reading lazy.

[Jsdirt]

Yeah, that's a "lean, bank 1" code.

 

Your post just reminded me of the test I used to do for this. Full throttle in 1st gear (so you don't end up doing 60 in a 30 zone ...), watch o2s on the scan tool. If they peg lean, it's starving for fuel. MAF should also be deep into the 100's of grams per second range as well.

 

And the MAF should read close to engine displacement in liters as grams per second (so a 5.3 should be near 5 or 5.3 g/s plus or minus a few tenths) at idle.

Edited February 4 by Jsdirt