the truck starts up fine and idles fine. about half of the time when you go to lower gears because you are slowing down the rpms do not drop. i will be setting at a light and my rpms will keep going up without me pushing on the gas. i have already cleaned it with carb cleaner and cant seem to figure this out. i check the throttle cable and it is not getting stuck what could this be.

 

WHICH truck?

 

Hey there mate,
The computer controls the IAC which in turn controls the engine idle.
Often these kinds of fault are linked to the speed sensor signal that needs to be active on decelleration.
On decelleration the computer uses the speed sensor to tell it what to do with the idle and fuel control when no throttle is applied.
This does several things. Shuts down fuel to a low level as it is not needed and saves gas. Closes down the IAC just enough to prevent engine stall.
Decell. also is the time the computer does internal table updating and testing when the needs to control the engine are the least.
This is info for you to relate to. What the problem will be, you have to search for.
I would be sure the IAC is clean and does not stick in any position, clean the throttle body for varnish and be sure there are no intake air leaks.
All of these will affect the idle to one degree or another.