Random misfire, strange fuel trim values

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by Emptyheadtech, Mar 20, 2014.

  1. Emptyheadtech

    Emptyheadtech New Member

    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
    In the last 20K : plugs, wires, fuel filter, fuel press reg. intake manifold gasket. All o2 sensors were replaced as well as a software update from the dealer.

    I have an intermittent P0300 with no other codes. I checked for vacuum leaks- nothing found.
    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 to possibly rich condition while LTFT indicate a lean condition
    the numbers seem to contradict themselves.
    I'm stumped....
     
  2. silverado002

    silverado002 Active Member 100 Posts

    I had a similar problem on my 2002 5.3L. (165,000 mi) Intermittent P0300. I did all the stuff you did too. I then started the truck and checked the wiring to the injectors and found that if I wiggled the injector wiring on the drivers side cylinder #3 and 5 it would set the misfire code P0300. I surmised that the failure might be due to vibration since the failure wasn't there all the time. I also found a coil pack with the same issue on #5. I took all the injector connectors off and cleaned them with CRC MAF cleaner and did the coil pack connectors as well. So far so good. This may not be your issue but it could be good to check. BTW my fuel trims are LT - 0.0 and ST -1.5 both banks.
     
  3. j cat

    j cat Rockstar 4 Years 1000 Posts

    what is the fuel pressure ?
     
  4. Emptyheadtech

    Emptyheadtech New Member

    48 psi @ idle, 55 WOT
     
  5. j cat

    j cat Rockstar 4 Years 1000 Posts

    you put the correct fuel regulator in there ? 48 psi is too low.......53psi or more. with the e85 engines its 50 psi.
     
  6. Emptyheadtech

    Emptyheadtech New Member

    As far as I can tell, it was correct. but it is an aftermarket item.
    The book mentions installing a shut off valve to test the regulator/fuel pump output, anyone have a part number? or is it something that needs to be fabricated?
     
  7. j cat

    j cat Rockstar 4 Years 1000 Posts

    I would re check the part number ..they did use different fuel regulators and with non E85 the pressure need be 53 psi or more.. not regulating correctly.
     
  8. Emptyheadtech

    Emptyheadtech New Member

    put the old regulator back in and the fuel pressure @ idle is 53. Erased the codes.
    Time will tell.

    Thanks for all the help!
     
  9. j cat

    j cat Rockstar 4 Years 1000 Posts

    this is a common issue with these fuel pump regulators ....they say it is correct then the pressures are wrong !

    thats why I say and do check the fuel pressures after any fuel component repairs.

    post back how it went . good luck..
     
  10. silverado002

    silverado002 Active Member 100 Posts

    I agree. I checked Rock Auto and they show at least 3 regulators for a 2002 truck with pressures ranging from 50 to 65 lbs depending on how the vac tube connects to it. Some of the regulators don't show any pressure specs at all which makes me wonder what they do.
     

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