When I first start my truck it idles high around 1500rpm for quite a while. It does come down eventually. Also when I slow down at a light it idles around 1300rpm for a minute then kick down.
I have a 1998 ford ranger 4x4 3.0 liter v-6.
Why does this happen? thanks

 

I would say the IAC sensor. My 98' ranger 3.0 had all kinds of idle issues and a new IAC fixed them all.

 

Welcome to FTE.

As 00GT has suggested, the Idle Air Control (IAC) probably belongs on your suspect list, as it's a common problem part.

You might try removing & cleaning it with a plastic safe spray, like CRC QD, or MAF sensor cleaner & see if it helps. Cleaning may not last, it didn't for me, but was a good trouble shooting step.

It's located on the intake air tube ahead of the throttle body. It's job is to bypass air around the throttle plate at the command of the computer, to control idle speed when we come to a stop, or apply load to the engine, like putting it in gear, turning the AC on, ect. Over time it gets fouled up with carbon deposits, or hangs up from wear & messes up computer control of idle speed. Ford recommends replacement rather than cleaning, but I cleaned mine as a trouble shooting step, it didn't last a year, but when it acted out again I knew what to replace!!!!

You could also have other things wrong, like a dirty sticking PCV valve, or a vacuum leak to it, or elsewhere, an acting out speed sensor, ect, figuring in, so if removing & cleaning the IAC doesn't bring joy, look deeper for another problem.

If your of a mind to begin turning your own wrenches, the Search function is our wrench turning friend, so be sure to visit the "Tech Info" thread atop this forums thread listing page for lots of wrench turning help, including a nice post by Rockledge toward the bottom, having to do with the 3.0L pushrod Vulcan engine.

Do you have a CEL lit???? If so, have the computer scanned for trouble codes & post All of the code Numbers, as they can provide good trouble shooting clues. If you don't have a code reader, or scantool to retrieve trouble codes, most autoparts stores will scan the computer at no cost if the CEL is lit, or your having problems.

More thoughts for consideration, let us know how it goes.