I have a unique problem with the idle. early 86 with 89 aluminum heads.
I replaced the injectors, removing only the plenum, and disassembling the fuel rail to accomplish the injector swap. New o rings and new plenum gaskets about 3 months ago.
Last weekend, I removed the throttle body to replace it with another factory early 86 thottle body because when i turned on the air conditioning, the engine dies, suspecting the IAC as being bad.
Since I had another unit, I figured I would just replace the entire unit instead of switching IAC's from unit to unit.
I disassembled the replacement throttle body before installing it and changed all the gaskets, and plastic media blasted it. Replaced the water jacket, lid, and IAC gaskets. I coated the new gaskets with a red silicone film, and dipped the screws in red silicone as well.
Now, I installed the "new" throttle body, performed a minimum idle adjustment, the TPS was no good, and swapped the TPS's, and adjusted the TPS again...to .51
I get the idle to 400 RPM in idle, car in gear, shut it off, plug in the IAC, start the engine and the car idles 100 RPM too high with the IAC plugged in.
anybody got any ideas?
THe engine does not die when I turn the A/C on and off now, confirming the IAC problem, but the idle is high.
A/C off, engine warm, idle 650-700. A/C on, idle stays at 600.
Previously, it was at 500-550.
The old throttle body worked just fine as far as minimum idle goes.
The old throttle body is extremely carboned up, and I will disassemble it and plastic media blast it, and replace the gaskets, and, I guess, play with switching the IAC's if I have to.
But why does the idle pop back up ?

 

oops

 

The tps setting is supposed to be .54 which, if you set it there it will probably raise the idle a bit more so my guess is excessive throttle shaft clearance letting in too much air. If everything mechanical is right such as cables etc. then it HAS to be too much air getting in the engine.

 

The car is fixed pretty much.....................
three things fixed this:
1. cruise control cable misadjusted, pulling set screw off idle stop.
2. idle screw set too low with throttle body butterflies scraping T B.
3. readjusted TPS voltage to .475
A. engine idles at 600 with car in gear with A/C running
B. engine idles 625 with car in neutral with A/C running
C. engine idles a solid 600 with car in gear, no A/C

I blasted the original throttle body and changed the gaskets, lubed bushings, reinstalled.

Note:
I switched the IAC back because the clip would have been on the bottom, making it almost impossible to remove the connector with the throttle body bolted on.
I plastic media blasted that IAC until clean, and polished the seat surface on it.
So the original IAC is back in the car, and operating just fine.

 

Not sure if I follow. Is the car an Automatic and now idling at 600-700 RPM after the TB swap? Off the top of my head, that's where the idle should be.

 

Pretty much a crapshoot until you know what the ECM is targeting as idle speed; ie, you need to scan it so that you can see that number (along with actual rpm; IAC counts; CTS; and the a/c request).

 

I'm not sure if this is relevant to earlier models, but for my 90, the FSM has a reset procedure you might try.
Depress accelerator pedal slightly.
Start and run engine for 5 seconds.
Turn ignition OFF for ten seconds.
Restart and check for proper operation.

 

Well, prior to my fiddling with the throttle body, it idled at 550-600 and all was well.
Now that I've cleaned out the carbon, and reused the original pieces (TB, IAC,TPS) the idle is 40-50 RPM's higher still.
and the car is an automatic.
The problem with a high idle of even 50 RPM's is the jolt from the trans when shifted into gear.
The later cars had some sort of valve to bleed off the initial pressure to do without the driveline shock.

 

You probably did this already but just in case. Did you disconnect the battery terminal and let the ECM reset? ...WW