P0300 at idle when fuel pressure gets high? Possible?

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by nbssilverado07, May 8, 2017.

  1. xPosTech

    xPosTech Epic Member 5+ Years 1000 Posts

    I had to sneak a peek at your profile to see if you were dealing with any salt or de-icing chemicals. I think you're OK there unless you spend a lot of time at Big Bear.

    Even with the lack of salts I would check wiring between the PCM and fuel pump control module and on to fuel pump. The pressure rise isn't constant although it's repeatable. I've never had trouble with mine so I have no experience with this subsystem.

    You might be able to check the PWM signal to the pump that varies the pressure requested. The wider the pulse the higher the pressure. A VOM set to AC Volts should tell you something. Also I don't know for sure but I think the pump uses a DC motor. If it is then a 12 or 14 volt DC would give 100% pressure. ie if a 12V signal was shorting to the pump PWM line you would get max pressure.

  2. Another odd thing I noticed today. With key on engine off the pressure is supposed to be 60psi I think and hold right? Mine just went up to 72.2 and held it might of wanted to keep going up but I just turned the truck on since i panicked thinking maybe I'm doing damage. Truck still fired up like always
  3. j cat

    j cat Epic Member 5+ Years 1000 Posts

    if the fuel pressure was 70 PSI that would damage the injectors.
  4. xPosTech

    xPosTech Epic Member 5+ Years 1000 Posts

    I think I might consider replacing the fuel pressure sensor just outside of the fuel tank. You're getting either inaccurate fuel pressure readings or faulty pressure control.

  5. Ok I'm going to do a mechanical gauge at the same time I look at my scanner and I'll report back my findings see if they match.
  6. xPosTech

    xPosTech Epic Member 5+ Years 1000 Posts

    OK that should tell you if the high pressure reading or actual high pressure is triggering the fault.

    Edit: If the PCM thinks pressure is high it should start leaning the injectors so engine would be running lean. But if pressure is actually high leaning injectors would work until it couldn't lean any more and engine would be running rich.

    Last edited: May 19, 2017
  7. Just did the manual fuel pressure gauge along with looking at my live data fuel pressure reading and it's reading exactly the same key on as well as idling and leak down.

    What do you think it means that my readings are both accurate but still pressure goes high idling and back to normal if I press on the accelerator idling?
    Last edited: May 22, 2017 at 2:01 PM
  8. xPosTech

    xPosTech Epic Member 5+ Years 1000 Posts

    When idling does the engine go rich?

    Something is commanding the fuel pressure module to go high pressure at idle. It sounds as if the PCM is asking for full pressure of the pump. If it's not asking for full pressure the FPCM may be faulty or the pressure relief valve is faulty.

  9. Yes at idle my engine flashes as my fuel pressure shoots to 59psi then I give it a little foot on pedal at idle and my flashing goes away and my fuel pressure returns to normal. But then a minute later fuel pressure back up and light flashes. If i take it for a spin the light does not flash while im accelerating and driving just at a long red light the fuel pressure goes up and flashing begins.
    Pressure relief valve is on the pump right? This thing is crazy complicated I have a new fuel pump on order so I was thinking see what that does but I'm really suspecting the fscm.

    Also my wiring to the fuel pressure sensor is good I'm getting 5v reference and ground as well as 5v when I jump the reference and purple signal wire. I'm also getting 12v n ground at the fscm as well as 5v when I jump the signal and reference wire as well as 12v ignition at the fscm. Wish i had a thousand dollar scanner now haa.
  10. xPosTech

    xPosTech Epic Member 5+ Years 1000 Posts

    The FPCM receives fuel pressure requested data from the PCM and sends a PWM square wave to the fuel pump. The pulse duty cycle for 59+ psi is 100%, or very close to it. For ~30 psi the duty cycle would be 50%, or a square wave with the high portion equal in width to the low portion. With an AC voltmeter the pump drive would read ~6 volts if the Vcc were +12 volts.

    Some more searching turned up a FPCM recall.

    Check it out.


    Attached Files:

Share This Page

Newest Gallery Photos