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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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23,870 Posts
The TPS (throttle position sensor) signal voltage may be wandering a bit at idle. It should have a steady 5 volt input and 0.2-0.3v ground measured to the neg battery terminal. The signal voltage will be around 0.65 volts.
The voltage value itself isn't as important as the fact that it should sit at that set voltage 'rock steady' without varying at all. If the voltage is being 'tickled' up or down even a few hundredths of a volt, it will change RPM's.
Since the TPS voltage is believable to the PCM (it may think that someone is playing with the gas pedal) it won't set a fault code.
I've had to clean throttle bodies and replace these TP sensors before.
 

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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
Joined
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23,870 Posts
I would probably prefer it running so that you could tell if the high idle revs coincided with an elevated TPS signal voltage.
If it isn't acting up, the signal voltage may appear normal.
 
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