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05 trailblazer no throttle response after testing for fuel sender connection

Discussion in 'Chevy Trailblazer Forum (GMC Envoy)' started by krazyjhawk, Mar 26, 2012.

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  1. krazyjhawk

    krazyjhawk New Member

    We are working on my aunts 05 Trailblazer with the V6. It was having intermittent fuel sending unit issues so we replaced the fuel pump assembly. After a couple days had the same issue again where the fuel gauge went to empty for a few seconds. We were testing for connection under the hood with a power probe when things started going down hill fast. The engine went from running fine to putting itself into "power down" mode. Then while trying to figure out why it put itself into that mode the vehicle wouldnt start at all. Now we can get it to start but there is 0 throttle response and no feedback from the PCM. Any suggestions? Does the PCM need to be reflashed?
  2. moogvo

    moogvo Moderator 1000 Posts 100 Posts

    If your sending unit is malfunctioning, it normally makes the gauge go to "full", not empty. The gauge is basically an ohm meter that shows full when the circuit is opened with infinite resistance and empty at low resistance. I would not suspect the sending unit for this. It sounds more like a short circuit.

    What kind of pump did you use? If it is NOT an AC Delco or a Delphi, you probably got a bad pump.
    1 person likes this.
  3. krazyjhawk

    krazyjhawk New Member

    We used a Delphi complete assembly. I think there was a wiring issue with the sender. So if it looses signal it goes to full? Now the more pressing issue is while we were testing for signal with the pump we have opened up a can of worms with possibly the PCM. It doesnt want to run and has put itself into that safety mode. It wont start unless we pull the fuel pump relay out. once it is running we can put it back in but it runs rough
  4. moogvo

    moogvo Moderator 1000 Posts 100 Posts

    It STARTS without the relay?!

    When the sending unit connection fails, yes. it will go to full. If the float gets a hole in it and doesn't float anymore, it will go to empty.
  5. krazyjhawk

    krazyjhawk New Member

    Yeah, this is quickly becoming a nightmare LOL. Any info on the power down mode it is in no? It has 0 throttle response, and wont even turn on unless you pull the full pump relay.
  6. moogvo

    moogvo Moderator 1000 Posts 100 Posts

    It sounds like you have some sort of wiring problem. With the pump relay out, it shouldn't even start! (of course, my Bronco II had 2 pumps... but the same relay powered both of them...) Hard to say with the newer stuff. The pump shouldn't operate without the relay though!
  7. krazyjhawk

    krazyjhawk New Member

    yeah, it is flooding itself out so we pull the pump relay to manage the amount of fuel going to the engine. It will die shortly after without the relay plugged in and then it idles horrible.
  8. moogvo

    moogvo Moderator 1000 Posts 100 Posts

    Oh. okay, so you are getting too much fuel when the pump relay is in place and it is just burning off the excess when you pull it. When you put your foot on the throttle, does it run right? If it does, check your IAC valve. If the pintle in the valve is all the way in, you are not getting any air at idle and it would flood out.
  9. Pikey

    Pikey Moderator Staff Member 1000 Posts 100 Posts

    I know that it is a long shot. But, have you tried disconnecting the battery for a while? While probing around you may have given the PCM some crazy readings causing it to go into power down mode. I don't remember who the poster was but they said that if you disconnect the positive AND negative cables from the battery and then touch the CABLES together (not the battery terminals), it will discharge the small charge that the PCM holds and clear any codes. I have not done it. I usually leave my battery disconnected for a few hours and it clears what ever code I have. Another thing, If it is flooding out when you try to start it, hold the gas pedal to the floor and crank it, I know with my 02 it caused it to enter "clear flood" mode and would no longer pump fuel into the motor until you let your foot off. I would guess that you don't need a PCM reflash. I looked around a little on the web and found that alot of people have issues with the o2 sensors on the Trailblazers that cause thew truck to enter into "power down" mode. There was also a PCM update from GM that is supposed to help with the issue. But, with yours showing up right after you poked things with a probe I doubt that it is your issue. You have something else going on. Check out #39 in the link:
    http://townhall-talk.edmunds.com/direct/view/.f1580ce/29
    Last edited: Mar 30, 2012

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