help me diagnose this Tahoe...

Discussion in 'OT Driven' started by Jesse_Boyer, Sep 28, 2006.

  1. Jesse_Boyer

    Jesse_Boyer New Member

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    for the life of me, I can't figure this out...

    1998 Tahoe, 2-door, 125k miles

    "Truck runs well cold. Starts great nice smooth idle.
    Truck warms up still runs great for the first 10 minutes plenty of power etc. After a total of 25-35 minutes of run time the truck surges terribly at idle, and has no throttle capabilities. -Not drivable.

    Truck surges:

    RPM Should hold at 625
    It almost dies then ramps up to 800 or so.

    Base timing should hold 0 degrees.
    It holds 0-1 when cold fluctuates up to 7 when surging.

    O2 sensors should fluctuate .2-.8.
    Primaries fluctuate somewhat when surging the post cats stay low often at .000

    Fuel pump should run at a constant speed.
    Hear it grind down as if the system is up to pressure-similar to when turn the key on before starting.

    Fuel pressure should remain at 55-60psi.
    It holds 55 psi when cold. It shoots to 60 then down to 40's when surging at idle. It jumps to 60 then flutters in 40's when trying to hold throttle.

    Exhaust does not seem to plug up, seems to flow OK at tailpipe. ( I have no official way to gauge at my house)

    Motor/Radiator has about 1000 miles
    Distributor/Cap/rotor/wires/plugs/#8 Injector have less than 200 miles.
    Issues started here before installation of below.
    Fuel pump/pressure regulator/fuel filter/fuel injectors have been cleaned all have less than 50.
    Crank sensor and relearn have less than 10 miles.

    Chevy dealer has been looking at it for 3 days without being able to diagnose the problem.

    Thoughts:

    Fuel pressure regulator is run on vacuum in intake. The map sensor holds a steady 10" hg when cold. Fluctuates 7-10 when surging."
     
  2. affende

    affende Resident 4X4 Elitest Prick

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    in your balloon knot
    have you checked the throttle position sensor?
     
  3. Jesse_Boyer

    Jesse_Boyer New Member

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    Dealer did, as far as I know.

    They are saying it's the cam sensor located in the distributor. (coming from my buddy)
     
  4. affende

    affende Resident 4X4 Elitest Prick

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    in your balloon knot

    could very well be....definitely sounds electrical though...sensor of some sort.

    this is why i work on jeeps and older toyotas....simple as shit.
     
  5. JRock10

    JRock10 Active Member

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    check alternator voltage. you could be dropping way too low to run all the electronics. if you're dipping under 10v, then that is probably your troubles. what's going on is the battery is building up a bit when vehicle isn't running, so you've got plenty of reserve power for that first 10 mins, then everything starts pulling on the battery. the alternator isn't working properly to maintain the demand of the electrical system, so everything starts shutting down.

    that's my theory.
     
  6. CastorTroy

    CastorTroy New Member

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    See if the dealer can reflash/swap the ECM. So many issues there that must be related that I would start at the brain of the system to make sure it isn't the problem.
     
  7. Jesse_Boyer

    Jesse_Boyer New Member

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    turns out the cam sensor in the distributor was the culprit. Drives like new again :)
     
  8. JRock10

    JRock10 Active Member

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    nice. see, there were many possibilities!! :eek5:
     
  9. dr.zed

    dr.zed DR.ZED OT Supporter

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    Cam sensor in the distributor???? Sounds like an LT1, which I know it is not, unless it is a transplanted engine.
     
  10. CastorTroy

    CastorTroy New Member

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    That's the one thing I dislike about modern computer managed vehicles, one faulty sensor and the whole thing goes to hell. Good it was something simple though.
     

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