My fiero was dying every time I put it in reverse or drive. So I put in a new IAC valve and it did it’s job the first day.... Next day I started it I tried backing out and it died again. I can only go in reverse when I have a foot on the gas pedal and brake pedal. It doesn’t die as much in drive which is really weird. Could it be a vacuum leak?

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Kaiden Meade

No.

Lets start by seeing if the IAC is working. Can you adjust your IAC by depressing the spring and screwing in the pintle? If so screw it in a bit and then reinstall it. The engine should start and run fast at first and then the ECM should reduce the idle by adjusting the IAC out. If so the IAC circuit is working correctly and we can look elsewhere. If not, then the IAC circuitry needs to be looked into.

To expand on phonedawg's guidelines, note that there are 2 styles of IAC's. The pintle shaft is the difference in how you adjust the IAC. If it has a round shaft, you can retract the spring from the pintle and screw it in as Timm describes. If it has slots on the side of the shaft, you must put pressure on the end of the pintle while rocking it side to side. Rotating it will damage the IAC and make it inoperative.

Each style should be adusted to 1 1/8 inch or slightly less as measured from the shoulder where the gasket would sit to the tip of the pintle. I just replaced one of mine and it finished the adjustment very quickly upon startup.

Sounds like an automatic. There is a torque lock up switch that might be a problem. If you unplug it and it improves, replace it. if not, replug it and keep on diagnosing...

😕 "Adjusting" the IAC does Nothing except make sure you don't damage them when installed.
If the tip is too long, will "slam" the seat inside when installed and jam or often wreck the "motor."

Think more like brake piston compression/retract when installing new pads etc. And Two style valves have different direction to "retract" the valve.

If you didn't bind/damage them, the valve will move and "reset" every time you drive over ~ 35mph. After "reset" then can "see" it move in ECM scanner as IAC Step Count in ECM data stream. For Auto Cars, just moving shifter slowly from P R N D and back will move the IAC. Stick cars, carefully load the clutch a bit should do same but don't drag a clutch very long and heat up or burn that.

IAC is 2 phase Stepper Motor driving the valve in/out.
"They" make IAC "Noid light" to see if wiring and ECM works. Is sim to Noid lights for Injectors but have 2 lights that blink as each phase is turned on & off as ECM tries to move the valve.
Note: Year and engine of car often matters because most Noid lights and more have plugs to fit a car.
For Fiero, 84-86 L4 and maybe all V6 have Weather Pack plugs but 87+ L4 uses Metri-Pack plugs for many things including IAC.

The IAC could be "bench tested" but you have to how stepper motors work plus make sure the valve can't move too much or can pop out when turn in close direction and can be very hard or impossible to reinstall the valve part in the motor. "They" make a tool for this but cost a lot of money for tool you likely use 1 time. Unlike Noid Kits have many Noids at HF etc costing <$50. 1 Noid often < $10.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 05-23-2021).]