GMC Savana Condemned PCM

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  • just2damfast
    Junior Member
    • Mar 2010
    • 8

    GMC Savana Condemned PCM

    2006 GMC Savana van, the other technician had misfire problems and replaced the maf only to find no improvement. I connected the escan to find very negative trim on road test. However vehicle intermittently running rough. Performed volumetric efficiency test and also found numbers off. connected vp and first used transducer on fuel rail, back probed, cyl. 7 injector and the cam sensor. found fuel pressure to drop and rise while running rough, cam signal good. But injector appeared to be firing a second time during compression stroke. Back probed cam and crank to check to see corallation. Seemed good. Probed cyl. 1 injector, crank signal (24x), and trigger of cyl. plug wire, cyl. 1 injector fires twice - posted below. If my theory is right, I believe the pcm to be bad if both cam and crank signals are good. Still new to the lab scope and thought I would post my findings.

    Brian
    Attached Files
  • crowbar
    Junior Member
    • Sep 2008
    • 7

    #2
    I don't know if this helps, I work on motorcycles.
    Some models fire the injectors twice, each time giving the cylinder a half load of fuel.
    I knew once why they did this, but I can't remember why right now.
    Could be because some motorcycles rev to 15,000 RPM.

    Comment

    • Joe Rappa
      Snap-on DSD
      • Aug 2007
      • 2050

      #3
      I don't think I'd ever condemn a PCM on a GM vehicle for delivering an asynchronous pulse. It's common practice for them.
      Joe
      "You don't build a reputation on what you're going to do"
      Henry Ford

      Comment

      • just2damfast
        Junior Member
        • Mar 2010
        • 8

        #4
        However, the extra injector pulse only shows while the vehicle is running bad. It's somewhat of an intermittent condition. When it runs well, the second pulse isn't there.

        Brian

        Comment

        • just2damfast
          Junior Member
          • Mar 2010
          • 8

          #5
          Joe Rappa, you were correct. replaced PCM only to find vehicle still running bad after about 3 hours of hunting, Low reference wire going to throttle position sensor was found broken inside the insulation. I wonder if this could have been the reason to the extra injector pulse. Perhaps for accelleration enrichment to keep the engine from bogging?

          Brian

          Comment

          • Joe Rappa
            Snap-on DSD
            • Aug 2007
            • 2050

            #6
            Originally posted by just2damfast
            Joe Rappa, you were correct. replaced PCM only to find vehicle still running bad after about 3 hours of hunting, Low reference wire going to throttle position sensor was found broken inside the insulation. I wonder if this could have been the reason to the extra injector pulse. Perhaps for accelleration enrichment to keep the engine from bogging?

            Brian
            I bet you're correct. Small increases in TPS lead to big increases in fuel enrichment. GM likes to add that extra fuel with Asnychronous pulses. Most everyone else increases injector on time.
            Great find!

            I was thinking you were on the right track with the cam/crank signals, but a bad TPS signal will definitely cause your problem

            Thanks for letting everyone know.

            Joe
            "You don't build a reputation on what you're going to do"
            Henry Ford

            Comment

            • vasau
              Senior Member
              • Jul 2007
              • 119

              #7
              What size engine or code?

              Comment

              • just2damfast
                Junior Member
                • Mar 2010
                • 8

                #8
                It is the 4.8 liter v8

                Comment

                • eddie8
                  Senior Member
                  • Mar 2008
                  • 273

                  #9
                  By the way just2damfast, how did you get the escan screen shots saved?i have the escan and cant seem to save my screen shots Any hints?
                  Edwin Hazzard
                  Southeast Mobile Tech

                  Comment

                  • Joe Rappa
                    Snap-on DSD
                    • Aug 2007
                    • 2050

                    #10
                    Hi Ed! Hope you're doing well.
                    Just use a screen capture program. I use MWsnap. It's free and doesn't use much memory.

                    I'm not scheduled to be back down in the Newburgh area until November, but I'll be sure to look you up when I get there.
                    What a crazy schedule I've got this year! Flying to 5 different cities in 5 days this week.

                    Joe
                    "You don't build a reputation on what you're going to do"
                    Henry Ford

                    Comment

                    • just2damfast
                      Junior Member
                      • Mar 2010
                      • 8

                      #11
                      Just hit F11, it's the shortcut key to save what is on the screen. I use this function alot to compare data before and after repairs. You can also save the screen by selecting one of the taps on the left side of the screen and hitting the "save screen" button. Hope this helps.
                      While we are on the topic of the escan, Lately I seem to be having problems connecting to chevys around the 2004 model year, does your's have any problems? The security light comes on in the car and the escan just won't connect. Also doesn't seen to like audis and vw at times. Just curious if anyone else was having similar problems.

                      Brian

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