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7.4L 454 big block not starting or running with gas

Discussion in 'GM Powertrain' started by Crawdaddy, Nov 19, 2010.

  1. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    My uncle's 1999 GMC 1-ton 7.4L big block dually doesn't want to start and run off of gas since he's rebuilt it. My grandfather has experimented and got the engine to run off of propane that's getting dumped into the intake at the vortech spot. He's replaced the fuel pump with no change. My grandfather and uncle tested the injectors by checking their resistance and whether the fuel rail pressure went down when the injector was fired. They determined that one injector is likely dead, but that shouldn't keep the whole engine from starting.

    Someone suggested to my grandfather that the timing is off by a tooth or 2, so they repulled and restabbed the distributor with no change. An OBDII code reader/scan tool reports that timing is around 19 degrees advanced, which people have said is too high.

    He's about ready to hand the issue over to a shop. Anyone have any idea what's wrong? Thanks for any help.
  2. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    We still haven't been able to figure it out. There's got to be something simple we're missing, we just don't know what it is. Any ideas we can go off of?
  3. aficio400

    aficio400 New Member

    I do not know much about the newer stuff. But have you checked for fuel flow before it gets to the injectors?
  4. stephan

    stephan New Member 1000 Posts

    I agree with Aficio. It's got to be smth in the fuel delivery cuz it runs on propane which eliminates the ignition as being at fault. Check the fuel pressure & see if the injectors are triggering.
  5. 2COR517

    2COR517 New Member 1000 Posts

    What's the fuel pressure?
  6. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    The fuel pump is new. The pressure at the shrader valve at the front of the fuel rail was around 80PSI even while cranking. They determined 1 injector may not be opening by applying power to each injector and checking to see if the fuel pressure on the rail went down with the ignition off. However, that injector being dead shouldn't be keeping the motor from starting. I keep telling them it's something in the fuel system, and at this point it's likely the fuel pressure regulator, but he doesn't seem to want to entertain the idea.
  7. 2COR517

    2COR517 New Member 1000 Posts

    80 psi is way too high. Not that it won't work, but something is wrong. Does it have the new MFI spider, or the old poppet injectors?
  8. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    Without fully understanding those terms, from what I do understand, it has the poppet injectors. It has 8 individual injectors about the size of your average lifter that plug into the intake individually. Would the too high of pressure mean the fuel pressure regulator's bad? That's been my latest guess for a while.
  9. 2COR517

    2COR517 New Member 1000 Posts

    It's either a bad regulator or plugged return line. Either way, fix it soon or you'll be buying another pump.

    The poppet style injectors have just a plastic line running to each poppet. The MFI spider has a tube along with two wires for each cylinder. The actual electronic injector is at the intake port. You should be able to tell by looking through the throttle body.
  10. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    From what my uncle's told me, he was getting 60ish psi after changing the pump. The fuel rail appears to be a straight plastic tube that has holes that connect to the injectors.

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