If you have a check engine light you may want to see what the hard code is setting, which can be checked by a technician with a scan tool. If there is no check engine light it could be a number of things such as, idle air control valve, throttle position sensor, or a few others. I would have it scanned and inspected by a professional.
There are many "things" on your car that can make the idle fluctuate up and down. Does your truck feel like it wants to stall? Is the engine light on? Take notes of what is going on when you start the truck. Vacuum and sensors are what pops into my mind first. If the engine light is on, that will help to determine the cause, and help to find a cure. Check out the list of female friendly repair shops on the askpatty.com site. They can help you for sure.
Thanks for writing in,
〉 Answered on Jun 23rd, 2010 by Audra Fordin, Owner at Great Bear Auto Shop