my 1999 v6 4.2l f150 has close to 200000 miles on it and is starting to act up. cranks okay and idles down to 850 rpm almost right after starting but when im going i push in my to switch to the next gear it will rev up to 3000 rpm sometimes mostly when its hot outside or ive been driving for awhile, and wont idle down until i come to a complete stop then it stays at 850 rpm until i have to shift. what are ya'lls ideas on what the problem could be and an easy, cheap, and right way to fix it?

 

do u have a cel light on? these engines have wahts called an iac motor. which controls idol according to engine load. and shifting it into gear puts a load on it could be giving an iac a weird signal. verify that the iac is working properly

 

Also check for vacuum leaks. A leak can cause IAC to operate in an abnormal position as it tries to compensate. You can remove the IAC and inspect it and clean it.