i have a 93 3.9 liter dakota with about 143,000 miles.....i have an idling issue.it sometimes will idle at the 4th bar 3rd bar and sometimes even 2nd...also as my truck changes gears(automatic) the rpms go crazy....i had an issue wiht a vacum line not being hooked up and fixed that then been cleaing out the system with seafom...what could be the problem?im thinkin maybe a sensor..o2 possibly

 

It could be a faulty idle air control (IAC) motor. This is mounted on the rear of the throttle body.

 

the weird shifting sounds like a bad Vehicle Speed Sensor (VSS) which may also affect your idle at a stop. But if its not that then the AIC is also a suspect as dodgerules86 pointed out.

 

i just changed the aic about a week ago and it kind of fixed that....where is the vehicle speed sensor located and how much does it cost

 

its on top of the rear end or on top of the tranny, can't remember for sure and dont have a chiltons with me atm.

 

And that controls what exactly?

 

It tells the computer if your moving or stopped, helps prevent stalling when you pull up to a light and does some other stuff I dont remember right now. Think it also tells when to shift too maybe, or something related to overdrive perhaps? I know a bad one can cause the vehicle to shift up and down sporadic which would explain RPM jumping you mentioned. This is all going off a very bad memory of reading I've done in the past. Should have mentioned before that I remember reading that if you VSS is bad then your speedo will say 0 until your over something like 25 or 30 mph. Does your speedo act weird? Also cruise control wont work if VSS is bad, IIRC.

 

I have cruise control i dont kno if it works honestly b/c it never has....my speedometer seems to work fine it just that the rmps with be up there and its still taking forever to get going

 

In an attempt to troubleshoot a problem I'm having I disconnected my coolant temp sensor (to tell me if O2 sensor is culprit) and with it unplugged I have same weird idle you have so I'd look at that too. You can test its resistance with a volt/ohm meter. See the thread about "Changed computer, now backfires" where dodgerules86 posted info on what resistance should be seen for what temperature the coolant is.

 

i ran some more seafoam through it today and it seems to be running pretty good,maybe ill run another can trhough it just for fun...it seems to be good for like 3 days then its back tho.its really strange