high idle when cold (Page 2/2)
Patrick DEC 19, 06:51 PM

quote
Originally posted by Williamm65:

Witch wire do you cut into



No sorceress was harmed in the modification of my Fiyero.

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All kidding aside, it was 25 years ago I did this mod to a Fiero that I stopped driving ages ago. Whatever wire it was in the IAC harness, when the circuit was broken, the IAC would not move. A schematic diagram should show which wire it is.

[This message has been edited by Patrick (edited 12-19-2023).]

82-T/A [At Work] DEC 20, 09:12 AM
Your problem might be related (somewhat) to the "thermac" system.

Sigh... I just did a search for THERMAC and Fiero and got nothing but pictures of Fieros. Ugh... so anyway, I'll try to explain it as best as I can.

There is a little sensor (usually blue or white) that's located inside the air cleaner. It has TWO vacuum ports that protrude through the bottom of the air cleaner. One goes to what's called the "THERMAC VALVE" and the other goes to a vacuum source (either the intake manifold or the throttle body, I can't remember). This in totality is called the THERMAC system.

This little blue sensor inside the air cleaner either opens or closes depending on the temperature of the engine and incoming air (basically). As the air coming into the engine heats up, or the air cleaner itself has warmed up, it will either close or open the valve (I forget which) which allows control of the large vacuum controlled flap at the very mouth of the air cleaner (as you see here):




A lot of people don't realize it, but that valve at the end of the air cleaner has a rubber diaphragm much like the EGR valve... which can dry out and fail. I'm not aware of any way to replace / fix this.

Anyway, what this does, is... when the engine is cold, it activates the valve at the mouth of the air cleaner and instead of getting air from the side / cold air intake of the Fiero, it instead gets air through a tube that should be connected to tinwork that's attached to the exhuast manifold... which in turn would be drawing in hot air that runs past the hot exhaust in order to warm up the motor more quickly. Here's a diagram I found on the internet for a generic GM configuration:




Why I mention this is because it could be ONE source of your problem. The 4 cyl Fiero's idle is naturally high... it's not uncommon for the Fiero to race up to 1,500rpms to warm up. But 3,000 rpms is definitely too high. It's possible you have a couple of vacuum leaks, and this is definitely one likely culprit. A vacuum line that size wouldn't make a huge difference, but could mean the difference of 100-200rpms. Either way, checking this system to make sure everything is good and operating properly can help your car warm up faster and reduce the cold rpms somewhat.

I'd also recommend switching to full synthetic. These motors take a ton of wear when cold.
Patrick DEC 20, 04:21 PM

quote
Originally posted by 82-T/A [At Work]:

Your problem might be related (somewhat) to the "thermac" system...



...keeping in mind that this system is only used on '84-'86 2.5 engines. It's not used on '87-'88 dukes.
82-T/A [At Work] DEC 20, 06:12 PM

quote
Originally posted by Patrick:

...keeping in mind that this system is only used on '84-'86 2.5 engines. It's not used on '87-'88 dukes.




I was responding to the original poster, who has an 84. I didn't realize it was from 2 years ago...