Random P0300

[Zombiewoof]

Hey folks- Hoping someone can shed some light on this one. I'll be as brief as I can:

 

2001 Silverado 5.3 120,000 miles with an intermittent P0300, with no other codes.

Fuel pressure is 56 psi @ idle with a solid 19"hg. in the last 20k miles I have replaced:: plugs, wires, fuel filter, fuel press reg. intake manifold gasket. All o2 sensors, and knock sensors.

despite all this, the P0300 still shows up, and my fuel mileage is down 25%.

I have a scan tool that is able to monitor fuel trim values:
at all RPMs/loads:
bank 1
LTFT +15-21%
STFT +/- 5%
Bank 2
LTFT +9-12%
STFT +/- 5%

STFT seem normal while LTFT indicate a lean condition
the numbers seem to contradict themselves.
I'm stumped....

 

 

[oldscsc]

You said it. Lean condition.

 

I'd be checking for intake gasket failure or a vacuum leak. The engine is sucking in unmetered air somewhere and the computer controls are (falsely) attempting to compensate for said condition.

[Zombiewoof]

wouldn't a vacuum leak skew the STFT numbers high?

My understanding of fuel trims is that the long term numbers are averaged out based on the short term numbers. since the STFT are staying within spec at all RPMs/loads, it does not make sense. Unless I am looking at this wrong....

I have been around the intake with the propane and found nothing; tirelessly gone over all hoses and tubes for cracks to no avail. would my vacuum reading be showing what it is if there were a leak? (19"hg)

 

Thanks!

[trapperman83]

I have found you can see it misfire on the scanner if you do the carb cleaner trick and have somebody watch your scanner, do u think it's possible your intake is cracked.

[8.1HD/ALLISON]

Short term fuel trims will dictate the long term numbers, but once it gets truck running efficiently the long term can stay high and the short term will level off. Does that make sence? Hard to explain. You might also look into having injectors cleaned(doubtful) , maf sensor issue, 02 sensors? Couple things to check.

 

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