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Thread: 2004 GMC C8500 7.8l limp mode

  1. #1
    Join Date
    Aug 2020
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    United States N.C. New Bern
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    Default 2004 GMC C8500 7.8l limp mode

    Got a 2004 gmc c8500 with a po1433 code fuel pressure sensor circuit high voltage and it starts bucking and goes into limp mode and throws a p1087 code fuel rail pressure trim top low you shut the truck off and start it back up the po1433 is still there but the p1087 is gone and it’s out of limp mode until you drive it again any help would be awesome

  2. #2
    Join Date
    Apr 2001
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    CA
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    Welcome aboard!

    Check for leaks and loose/damaged electrical connections, as well as the fuel supply system (replace filter, check for line damage, fuel quality, air intrusion, etc.). If you have access to the service manual, it's best to follow the DTC diagnostic flow chart, which will identify a failed sensor or other component early on.

    Otherwise, it sounds like a simple sensor (rail pressure) or communication/connection failure, but it should be verified with a scanner. Compare default commanded and actual pressures, then increase idle rail pressure and verify the actual pressure follows within the margin. If pressure values fluctuate greatly, or are absent, it's likely the pressure sensor, however, with a high circuit fault, it's more likely the sensor itself (sounds like it, due to your stop/start description). If pressure values are steady, but unable to increase or stabilize at higher pressure, or flattens at higher pressure (non-linear increase), it's likely the rail pressure regulator (it could be a pump issue, but save that for last). Normally, I'm opposed to just throwing parts at a problem, but these types of problems are more straight forward. The HPCR fuel system is fairly simple (not many active component parts), and there's a significant cost difference between sensors and mechanical parts. It's certainly worthwhile to replace suspect sensors before considering a costly high pressure pump.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    2014 Chevy Cruze Diesel - Fabulous car, no problems at all, but sold Nov. 2016 @ 55K miles.
    dmaxmaverick@thedieselpage.com

  3. #3
    Join Date
    Aug 2020
    Location
    United States N.C. New Bern
    Posts
    2

    Default 2004 GMC C8500 limp mode

    It started out with a P0335 crankshaft position sensor a P0091 fuel pressure regulator solenoid 1 control circuit low voltage a P1291 the fault code is not found in data base a P0087 fuel rail pressure to low a P1087 fuel rail pressure trim top low a P0571 cruise control brake circuit a P1433 fuel pressure sensor circuit high voltage. Found a service bulletin on the truck online from Isuzu for the fuel rail pressure regulator so we hooked up our scanner and started going down their check list and they said to replace the regulator so this past Saturday we replaced the fuel pressure regulator and a new crank position sensor it runs a lot smoother but the P1433 code just won’t go away and within a few mins of driving it it goes into limp mode

  4. #4
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    CA
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    Did all these DTC's all show up at once? If so, and in general, when a bunch of unrelated codes appear out of nowhere, chances are the DTC's are not valid, but another fault in the system is causing erroneous coding. This can usually be traced back to a single event, such as bad battery(ies), flaky alternator, improper jump-start, system short, excessive voltage (such as improperly connected multiple batteries), loss of a ground (lose a bonding strap, and the system will ground through other components, including the PCM), or collision damage. The PCM is also suspect. They don't fail often, but they do. Often when in the process of catastrophic failure, the PCM will dump erroneous codes, or seemingly erroneous codes that may be assembly line checks that are not listed, such as the P1291, which has no reference to the truck you're working on.
    Last edited by DmaxMaverick; 09-01-2020 at 12:58 PM.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    2014 Chevy Cruze Diesel - Fabulous car, no problems at all, but sold Nov. 2016 @ 55K miles.
    dmaxmaverick@thedieselpage.com

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