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Fast Idle in Morning

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Hobbes68, Dec 7, 2000.

  1. Hobbes68

    Hobbes68 Registered Member

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    As it gets colder here in the Northeast my 90 Jimmy 350 TBI has been idling quite high (more than it used to last winter) when I first start it up. It's at the point that I have to let it idle for about 5 or 10 minutes before I can drive because the idle is up to approx. 2500 rpm (can't be good for the engine, or trans at stoplights). It doesn't happen unless the ambient temp is below 50 degrees or so and it seems to get worse as it gets colder. Someone mentioned that the Coolant Temperature Sensor (for the computer, not the guage) could be going bad. Has anyone had this problem? What did you do to solve it? Did the CTS correct the fast idle?
     
  2. DMK

    DMK 1/2 ton status

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    I would go ahead and replace it. It won't hurt mine was only
    4.99 at autozone.
     
  3. K5 NUTT

    K5 NUTT 1/2 ton status

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    Moose's 90 Burb was doin the same thing. Put your finger under the egr valve and push up on the diaphram...if the engine doesn't sputter and die its time for a new one...also change your O2 sensor every 30K...has nothing to do with high idle but it saves gas money! Also while you are at it remove the IAC motor and clean the pintle up with some carb cleaner and shot some in the bore to remove excess carbon.

    Azblazer

    1979 K5
    1976 K5 Chalet
    1972 K10 Suburban
     
  4. mudhog

    mudhog THEGAME Staff Member Super Moderator

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    i would just change it and also check the egr valve

    s.smith 77blazer lookin 4 mud
     
  5. realsquash

    realsquash 1/2 ton status

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    I don't think it's your CTS because nothing in the ECM tells the engine to run up to 2500 RPM. It is most likely a bad IAC or a bad throttle body gasket. Cover up the opening that the IAC uses to pull air from inside of the air cleaner and see if your idle goes down. Are there any error codes?

    Squash
    http://www.trailrunners4x4.org/users/realsquash
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Also check for vacuum leaks. It doesn't take a whole lot of extra air sneaking into the engine to really raise the idle speed when there's no load on the engine.

    <font color=black>HarryH3 - '75 K5</font color=black>
    http://ThunderTruck.ColoradoK5.com
     
  7. Gandolf

    Gandolf 1/2 ton status

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    Also check the mechanical linkage. At the TBI, manually pull the linkage in the direction of idle to see if it's hanging up somewhere.

    '89 K5 Silverado 4" lift w/BFG 33's
     
  8. Emmettology 101

    Emmettology 101 3/4 ton status

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    My 88 Van used to do this and it was the gasket between the TBI and the intake. My mechanic told me that he has seen many gaskets do that. Not too long after that and the one on my Brother '91 started doing the same thing so he replaced his himself and it quit.

    HTH

    Mike [​IMG]
    See <font color=green>EMMETT</font color=green> -&gt; http://emmett.coloradok5.com
     
  9. Hobbes68

    Hobbes68 Registered Member

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    Update to: Fast Idle in Morning

    I replaced the Throttle Position Sensor, Coolant Temp Sensor and thermostat. I also found that the previous owner had removed the idle adjustment screw plug so I can adjust the throttle position. Turned it down a few turns and it's like a new truck! Throttle response is crisp and the truck idles very well. It still idles high when cold, but no where near what it used to. Also it only does so for about 3 minutes before it's down to normal.
     

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