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Thread: 2.8L ideling too high

  1. #1
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    2.8L ideling too high

    I have an 86 S10 Blazer with a 2.8L TBI. It is idleing at high idle speed. When you first start it, it high idles, but it never really seems to slow down once it warms up. Also when it is warming up, it sounds like it's trying to idle down, but stumbles and revs the motor back up. Once the truck warms up, it doesn't stumble, but stays on high idle.

    The check engine light is off, and there are no codes stored. I'm thinking it might be a vacum leak. There's miles of vacum hose.

    Thanks
    Steve

  2. #2
    LQ4 m8w6r77's Avatar
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    thats what i was thinking. more and likely that is what it is.

  3. #3
    Senior Member bigjsp's Avatar
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    Idle air control motor.

  4. #4
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    Wouldn't a vacum leak give a trouble code. Maybe MAP sensor for low manifold vacum?

    Thanks
    Steve

  5. #5
    Registered User hizzity's Avatar
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    mine does the same thing and i have no idea what it is.

  6. #6
    Senior Member bigjsp's Avatar
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    Here's some tech talk that I found when I was troubleshooting my truck: see previous thread Blazin Low Forums > Engine/Performance > Engine & Drivetrain > rebuilt 4.3 won't idle down

    Idle Air Control

    When the accelerator pedal is released, the throttle plate in the air intake closes completely. Because there are three requirements for combustion to take place - air, fuel and ignition - the engine would stall. To prevent this from happening, a passageway is built into the throttle body that allows air to bypass the throttle plate when it is closed. To control this opening - and maintain a correct idle speed - the passageway must be controlled by a valve. This is done in two ways:


    Idle Air Control Solenoid
    A solenoid controlled valve is activated by the computer. Because a solenoid only has two positions - open or closed - the computer must open and close the valve at a predetermined rate to control air flow. The rate at which the valve is opened and closed is know as the duty cycle. The duty cycle will increase as more air flow is needed.

    Idle Air Control Motor
    A small electric motor is used to position a valve in the bypass passageway. The motor will vary the size of the opening by moving a pintle back and forth to increase or decrease the size of the opening, and control idle speed.

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