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Code 42"STUMPED"

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by SRIslash, Mar 28, 2012.

  1. SRIslash

    SRIslash New Member

    !989 GMC 1500 4.3 V6
    Well I'm hoping now that someone on here has ran into this.I'll start with my replaced parts,distributer module,computer,Used distributer,and lastly,Timing chain set. So the truck started acting like it is in limp home mode and is reluctant to go over 30-40 kph.If I put the throttle down it will most of the time flatline.Sometimes if I push gently on/off a few times it will accelerate better.If I disconnect the battery for a while it will act reasonably for a bit but go back to it's new norm.This has been going on for weeks and a code 42 seems can be anything spark related.I have done my best to hunt and clean any grounds to no avail. Could the MAP sensor cause this?Or anything else would be helpful.
    BTW gas mileage is great when you only put around.Hehe
    Thanks,Shawn
  2. SRIslash

    SRIslash New Member

    58 views but no answers, mmm,
  3. mfleetwood

    mfleetwood New Member 1000 Posts 100 Posts

    Is 42 the complete code? Could it be P0420?
  4. SRIslash

    SRIslash New Member

    No it's a 1989 truck. Just 42.4 blinks then 2 blinks.
    Which wire would be the bypass timing.That is the only thing I can think of.
  5. mfleetwood

    mfleetwood New Member 1000 Posts 100 Posts

    Odd code....I found a few threads for mid to late 90s and one for a 2000, but all had to do with the code P0420. I'll keep looking (here and on some of my favorite websites) and see if I can dig up anything for you. I have no idea about your wire question....sorry.

    ---------- Post added at 08:23 PM ---------- Previous post was at 08:18 PM ----------

    Here you go.....



    CODE 42
    Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system.
    During cranking, the timing is controlled by the ignition module while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.
    An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.
    The conditions for setting this code are:

    • System in BYPASS mode but the ignition module is still controlling timing

      - or -
    • Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.


    Typical causes for this code include:
    1) BYPASS line is open or grounded
    2) EST line is open or grounded
    3) PROM or CALPACK not seated properly in the ECM
    4) Poor connections between ignition module and ECM
    5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
    6) Faulty or incorrect ignition module
    7) Faulty ECM
    Last edited: Mar 29, 2012
  6. SRIslash

    SRIslash New Member

    It is that 1) Bypass line open or grounded that I am Questioning
  7. 08_rado_rocker

    08_rado_rocker New Member 100 Posts

    I know with the code it is giving you, this might seem a little irrelevant.. but have you checked your fuel pressure?

    This sounds EXACTLY like what happened to my work truck a couple of years ago. Everything from the restarting to it acting better when you only feather the throttle is tell-tale to me fuel issue. If your fuel pressure cant keep up with the amount of air you are trying to shove into the motor.. obviously it will attempt to die/fade out on you.

    If you can get a gauge on there and check it, then I would start there (meaning the fuel pressure regulator and/or pump)
  8. mfleetwood

    mfleetwood New Member 1000 Posts 100 Posts

    I just "borrowed" this from another site, so I'm not 100% positive it is accurate info, but the author seems to know what he/she is talking about. If this doesn't work, let me know and I will try and find you a diagram.



    Bypass line is the brown wire with a black tracer that has a in-line connector near the distributor and firewall that you disconnect to check the timing. This code will set by either unplugging the wire or if the EST module is faulty.... Check to see if that wire is unplugged....
  9. SRIslash

    SRIslash New Member



    I haven't found the "set timing" connector yet.I assume that is what that is.
  10. 08_rado_rocker

    08_rado_rocker New Member 100 Posts

    I thought the same thing when my work truck did that to me.. granted it was a 2006 FORD.. but it was the exact same symptoms as you're explaining. which could seem as if the vehicle was in limp mode AND would cause a spark issue, as the air/fuel mixture would be inadequate for the throttle position desired when you stick your foot in it.

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