7.4L tbi + fuel problems...injectors dump gas (full open delivery)

Discussion in 'Performance & Fuel' started by 808-Con3, Jul 21, 2011.

  1. 808-Con3

    808-Con3 New Member

    7.4L 1992 GMC sierra 1ton
    --Problems just started, i have to fully depress pedal to start
    --injectors have been replaced, they pulse during ignition...when started, injectors spray constant, dumping fuel
    --NO o2 sensors on the manifold

    **NO engine codes, only code 12
    **Can't figure out why injectors are not pulsing, idle revs up to 1,500
    --ECM replacement? TPS and IAC need to be replaced?
    --everything on the harness gets the correct Voltage.

    I'm thinking that the ECM needs to be replaced, the IAC and possibly the fuel pressure regulator.
    Very hard to maintain an idle now on the truck and the conical spray pattern is thick and constant when the truck is running.

    No MAF sensor or O2 sensors on this TBI 454 engine...it's a 1ton.
    again, i replaced the injectors but it's not fixed yet.
    Played with the idle screw to no avail as well. :grrrrrr:
  2. Bigg Ron

    Bigg Ron Rockstar 100 Posts

    Welcome to the club!
    Have you checked the crank sensor?
  3. Chris Miller

    Chris Miller Rockstar 100 Posts

    Your ignition control module sends out the signal to pulse the injectors. Have you replaced the stock distributor? If not, have the module tested. If so, put the stock distributor back in. If all that's good, you should check the harness from the module.
  4. 808-Con3

    808-Con3 New Member

    Thanks guys!
    Yes, i checked all the sensors but only a voltage check.
    Haven't checked the distributor yet.
    Stock everything. After i pulled the harnesses to the distributor from the ignition coil...truck wouldn't start. That's as much as i did with that yesterday.
    Sounding like the ECM computer might need to be the last thing to replace...
    I'm gonna recheck the harnesses....i only have a chilton's for this truck not the FSM and the voltage levels per harness aren't exactly listed well...
    I'm gonna guess the usual 12v or 4.5-5v??? Sound about right or do i need to locate a more specific level and drop or variation under load for this harness?
  5. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    Check the ignition module that Chris mentioned. It's under the distributor cap and goes out a lot. It's probably not the computer.
  6. 808-Con3

    808-Con3 New Member

    Solid! I'll post an update later today thanks again for the help so far.
  7. 808-Con3

    808-Con3 New Member

    Replaced the Ignition module, Throttle position sensor, and the Idle Air Control valve.
    No progress. Injectors still idle with steady stream of gas and idle holds around 1500 RPM.
    Only thing left is MAF sensor (maybe called MAP?) and the ECM right?

    Not sure what else would be signaling the injectors to pour gas.
    Is there a relay or something else along the chain that might be stuck or not operating correctly?
    I switch the under the dash relays around just for fun and that did nothing either.
    Truck gets harder and harder to start through out the day and holding the accelerator to the floor is a must for the truck to start at least 98% of the time.

  8. 808-Con3

    808-Con3 New Member

    MAP sensor is next...if that's not it, then the ECM...

    Crank pos sensor...Bigg Ron...where is that one...
    --Front drive side of engine or back tucked up by the starter on the passenger side? Didn't see anything else. Haven't had or heard any problems with the flywheel or starter though...heard starts and idle can be rough though if the crank sens. is out...
    After messing with the MAP sensor, unplugged and the engine light came on...ran better and raced up to 1500 when plugged back in...so makes me wanna rule out the ECM and crack sensor so far.
    Thanks again guys...always helps.
    First time i've ever worked on a TBI...
  9. stephan

    stephan Rockstar 4 Years 5000 Posts

    Hey Con, if your voltages all checked good & you see them pulsing during cranking then I think this is all good, ECU's good, & dist. module's good. They're just too fast for you to see them at idle. If you hook up a timing light & point it at the injectors & watch them at idle RPMs you should be able to see they're fine cuz it will cut the actual real time down because you'll only be able to see every 8th revolution. I think your high idle is something else, rather than injectors dumping.
  10. 808-Con3

    808-Con3 New Member

    Yeah, replaced the MAP sensor..problem persists.
    The plugs keep getting really fouled out with gas and are black drippy messes.
    I'm gonna clean the plugs again, check the timing again and go from there.
    --CRANK sensor...couldn't find it yet....really hard to see around the starter motor...I don't have a lift and it's 4x4.
    I'll do my best but my manual doesnt show a location so im going on past experience that it is located near the starter motor...anyone know where that is exactly?

    This sure is one of the harder problem finds that i've had in a while...kinda funny because i know it's something easy.

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