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Your SPEEDO is working properly?


Yep

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It does this only when you are driving (vehicle in motion)???

Yep

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And if you pull off the road when it starts this, and just let it idle?


It will drop to normal idle within about 6 to 8 sec or when it makes a total of about 6 or 8 fluctuations. But it will always stop fluctuating before I get stoped.

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Does it stop doing this the instant you crack the throttle open again?


Yep

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Does your truck have A/C?


Yep

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If so, are the hoses to the fast idle solenoid routed properly?


And yep

Steve C