I need help... and a beer. Can't firgure it out

Discussion in 'General Chevy & GM Tech Questions' started by tatasta, Aug 28, 2009.

  1. tatasta

    tatasta New Member

    My son is now the proud owner of my old 89 Chevy 2500 4X4. It has a TH400 and a 350, which have both been rebuilt. It has ~ 80k on it and has always ran very strong with zero problems... Until recently.

    A couple of weeks ago he called and told me his truck was running very poorly and that it had run fine the day before.

    It started out looking like a fuel problem but no so sure now. I can get it to run and idle fair IF I unplug the MAP sensor. If unplug the MAP and put it into gear, it will die.
    BUT, if I leave the MAP unplugged (Vacuum line only) AND disconnect the TPS, it will continue to idle once I put it in gear.

    I have replaced/inspected so far:

    Fuel filter
    TPS sensor
    Distributor(complete including rotor and cap)
    MAP sensor tested (I may have diagnosed incorrectly, however)
    Plugs and wires
    EGR is working

    As noted, I can get it to run smoothly in park if I unhook the vacuum to the MAP but if I put it in gear it will bog down and die.

    AND, if I unplug the MAP AND disconnect the TPS, I can put it in gear and it will continue to run. To the point I am pretty sure that I could drive it. But if I plug either of them back in, it will die.

    I could use any input on this one.

    Thanks in advance.
  2. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    The symptom usually points to a bad TPS. Since you changed it it may have a bad ground. The ground wire going to the thermostat housing can cause this problem. Clean that and any other ground wires on the engine. Also if anyone has had the heads or manifold off look for loosed or missing grounds.
    And last but not least, clean the battery cables as they can cause weird sensor problems.
  3. tatasta

    tatasta New Member

    Thank you for the great information. I will look over the throttle body again and check all grounds. The throttle body has been removed and "rebuilt" with a tbi kit. So it has been cleaned well.
    One thing that I do notice is that when the TPS and the MAP sensor are connected, the idle surges very bad and I can here alot of suction sound in the throttle body. It sounds like a really loud vacuum leak but it is the TB itself. When I disconnect the vacuum line to the MAP, it will rev up some, idle back down a little, run smooth(er) and the sucking noise will go away. At this point if I put it in gear it will die. But then if I unplug the TPS, I then can put the truck in gear and it will continue to idle.

    Why do you think that the truck responds to unplugging the MAP vacuum this way? It seems to be a problem between spark advance and choke/mixture.

    I also unplugged the O2 sensor last night and it appears to be working properly.

    Thank you for your time.

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