88 2.9 Jacked up Idle...need help bad!

[Diggerauditor]

OKay, reading some forum threads I realized I screwed up my idle screw, which is not really an idle screw but a TPS adjustment...sigh.

My truck has 172k on it and has run fine and still does. I rebuilt the top end about 30k ago, cracked head, but everything is fine. My son has been driving during the summer and came over saying that the idle was jacked up. The idle was in fact wacky so the first thing I did was remove spout connector and check timing, it was fine. Then I started moving the idle screw and thats where it all started.

My guess is the IAC was bad, and or the TPS. I adjusted the idle screw, and then adjusted the TPS and unplugged battery. No luck. Had to drill out TPS mounting holes to get it to adjust. This morning I replaced the IAC with one from my 91 Mustang 5.0, uses the same exact one, and the idle is still all over the place. I need help setting the idle/TPS screw back to where it should be and then diagnosing the IAC or TPS. Im leaning toward TPS since my mustang always drives great.

Any help would be appreciated. Thank you in advance.

 

[1989LongbedGT]

If you moved your TPS Adjustment then no wonder your idle will be high. The computer uses the TPS signal as a reference to where the throtle plate is. Adjusting the TPS changes your reference voltage to the computer.

TPS works on a 5 volt reference. WOT your voltage from your sensor should be approx 4.5 to 4.9 volts.

at idle it should be around .4 to .5 volts. If you adjusted your tps and the computer is seeing a voltage of about .8 or .9 it will compensate.

Best thing to do is find out what the values of your TPS is at idle (closed throtle) with a Digital Volt and Ohm meter back probed with the sensor plugged in. Watch your voltage as you adjust the sensor. Once your close to .5 volts or whatever the spec is...then tighten up the sensor! Simple! Should be 3 wires from your TPS. Power, Signal, Ground. Signal usually in the center and ground on either side. Computer will feed it a 5volt reference.

The values I gave you are just a generic value for most 5 volt sensors. Im sure if you googled it you could find out the specs

Try that. Also have a look around for any vacuum lines that may have popped off or cracked. A good ol vacuum leak can cause all sorts of issues.

If you need anything else just PM me

 

[Diggerauditor]

I set the TPS to be at about .985. I set my Mustang to that, idk if the TPS runs off of different values or not. I set the idle last night and it runs better. Unplugged the IAC and ran it for a few secs at about 1500-2000 rpm then let it drop. Worked good and it ran fine this morning except for a couple of times coming to a stop and it varied from 500-1200 rpm for a few secs. Is that still the IAC acting up?