Help..Engine speed too high

I have a 1985 Cadillac DeVille where the engine speed is too high. This vehicle will reach 40 mph without even pressing down the accelerator. The engine has recently had a tune up and there are no error codes
present.
What would cause this type of situation, and is there anything that I can do to correct this problem?
Thanks in advance.
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Dave
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The idle speed control motor on these were noted for failing, look at the brass colored solenoid on the l/s of the throttle body( rudimentary description)

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