it depends on the year. if it's older it could be a feature that makes the car warm up faster, or it could be a throttle sensor gone bad, or it could be your timing is out, or your air fuel ratio is off. plus many more.
Your 2000 4.8 liter Sierra might run rough at an idle because of clogged fuel injectors or a clogged fuel filter. You might also have gotten water in the gasoline.
Check/ replace the TPS if the idle is up and down.
The idle speed is computer controlled and is not adjustable.
Idle air control motor or IAC
5 to 6 hundred rpms. Is recommended idle
low idle and uneven idle
The idle is control by the idle air control motor from outputs from the Engine Control Module.
Idle is computer controlled and is not adjustable. If the idle is incorrect, you have a computer, sensor, wiring problem.
If it has EFI (Electronic Fuel Injection) there is no idle speed adjustment. The idle speed is adjusted electronically.
Clean the MAF sensor with CRC MAF cleaner, do not touch the sensor wires, you'll damage the unit and freah is ~ $175.00
"How do you move the idle tensioner for a 1998 3 litre dodge?"
there is a little screw on the back of the carb. if you loosen it, it will idle slower, tighten it, it will idle faster.