IDLE PROBLEM AT STOP SIGN

[Tin butcher]

I've got an idle problem with my '85. Coming to a stop sign, I push the clutch pedal and the RPM's go up to 1400. Ten seconds after coming to a complete stop, the idle drops down to where it should be. This started after the original IAC failed closed. I replaced it with one from my local NAPA store. There are no trouble codes. I have replaced all vacuum lines and checked everything I could think of. HELP!

 

[SPANISHVETTS]

Did you clean the TB and reset the IAC?
Have a look at this.

 

[Tin butcher]

Yes. I cleaned the throttle body, reset the tps and the iac. I had bought another chip so I put it in. No change. The previous owner appears to have replaced the entire EGR system. This problem appears to be tied to the speed sensor, so I disconnected the cruise control. No change. One modification I did was to throw away the 4+3 (which had a major failure) and replaced it with a Tremec TKO 500. Since the OD was gone, I removed the OD relay. I cut the wire ends off, taped them individually, and then tucked them into the wire loom. Looking at the wiring diagram in the shop manual, this should have no affect on the ECM.

 

[JonM]

I have the same problem and discussed it with c4c5specialist a few weeks ago. His diagnoses was old linkage to the throttle bodies. I know yours and mine are different, but your linkage is almost as old as mine. Just an idea

 

[Tin butcher]

Here is an update: Idle problem fixed! After much research, it turns out that GM programmed the idle this way. To fix the idle, I had to reprogram the computer - so the fun began! I updated the 870 ECM to a 165 ECM, bought a laptop computer with a serial and parallel port, bought a PROMinator chip from geekinavette (Speedtronics.net), downloaded TunerPro RT and an APYP bin file. After much reading from several different websites, a lot of trial and error (mostly error), I loaded a program - it actually ran! To adjust the program to change the idle, I located "Enable IAC PID if MPH is GT 1.88". I changed that to 55. Next I changed "Idle RPM VS Coolant Temp" to 750 rpm. I drove the car but every time I pushed in the clutch, it would try to die before the IAC would catch up. I then reset the mimimum idle speed to 700 rpm and the TPS to .54 volts. Now the car idles how I think it should!