Idle issues '87 STX

[El_Capitan]

So I am new to this forum, but I have owned my Ranger since 1992. It is an '87 4X4 STX with a 2.9v6 5spd with approx 250k on it. Now I have had this issue for quite some time now, wandering idle sometimes 850-900 sometimes 1500. I have replaced the Iacv the Tps sensor coolant temp sensor and switch, checked and set the base timing, I have checked and repaired all the vacuum leaks and I am getting a system pass code from the ecu. It so much better now, but still every once and awhile it acts weird again. I one point not relizing what I was acually doing I adjusted the the set screw on the throttle body. I have since tried to adjust it to spec 0.85 volts but then it idles at around 1800 and will not go any lower. I currently have it set around 0.76 volts and it runs fine.. My questions are: Is this normal? Do I need to set the tps at 0.85 volts then disconnect the battery to clear the keep alive memory? Or just say F-it and let it ride? Thanks for taking a look, and sorry in advance for yet another thread about this problem

 

[holyford86]

there is a specific procedure for setting the tps voltage, it has been outlined before. Pull codes, 87 trucks didnt have check engine lights.

 

[El_Capitan]

there is a specific procedure for setting the tps voltage, it has been outlined before. Pull codes, 87 trucks didnt have check engine lights.

As of right now there are no codes, well except for the system pass code. I have tried the setting procedure: Backpin the center connection on the tps ground on battery set to 0.85v.. result: idle way to high. I have yet to set it at 0.85v and disconnect the battery. I'm going to try that tomorrow and see what happens

 

[holyford86]

the exact voltage is somewhat irrelevant as long as it is within the accepable range of the sensor its fine. Check for vacuum leaks then set the idle using the following method:

get the truck up to operating temp and shut it down.
Unplug the IAC and start the truck (you may have to give it a little bit of gas to get it to start) run it at 2500 rpm for 30 seconds, let off and see where the idle is. If it's at 700 rpm its good, if its not, adjust until it is at 700 rpm.
Shut the truck off and wait about 30 seconds, restart and run at 2500 rpm for 30 seconds, see what the idle is, if it's at 700 rpm shut the truck off and reconnect the IAC, If not repeat the adjustment procedure.

I found this procedure in Shop Key after .9 volts gave me an idle that was way too high. I did this and it runs right at 850-900 rpm at a warmed idle

if the idle speed doesnt change with the IAC unplugged then you may have a cooked or stuck open IAC