It could be a number of things, vacuum leak, worn throttle body return spring, binding throttle cable, and handful of sensors ETC

Lets start by getting some basics from you.

What year is your truck and what engine does it have?

Do you have a check engine light on?

Does the truck have any known stored codes in the computer?

Does the check engine light even work with the Key on engine off?

How many miles are on the truck?

__________________
2002 Ford Ranger 2 Dr Standard cab 2.3 5 SPD 7.5 373 gears