I have an 84 2.5L and ever since I got it, it idles wayyyy too high. it's only when it's cold out (the colder, the higher. the coldest I've started it is around 30 degrees and it shoots up to 3k rpm.) when it's warm out, it starts and idles at like 1500 or 2000 which is still a little higher than I would expect, but not as bad as 3000 at 30 degrees right on start up. it comes down after the engine warms up.
Engine warm to hot idle should be ~ 800-900 RPM depending on trans w/ it.
Cold to very cold then 2000 - 3000 is common when started and soon after gets slower and slower til reaches normal idle. Is to keep engine running when Trans goes into Any Gear.
ECM uses ECT to set Cold Idle even when IAC is reset after driving > 35 MPH after discon the battery etc.
Use ECM scan tool to see ECT and other sensors output. If ECT says colder then weather low the last several hours then sensor or wires to it could have problems making engine w/ even higher idle then should.
------------------ 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)
Engine warm to hot idle should be ~ 800-900 RPM depending on trans w/ it.
Cold to very cold then 2000 - 3000 is common when started and soon after gets slower and slower til reaches normal idle. Is to keep engine running when Trans goes into Any Gear.
ECM uses ECT to set Cold Idle even when IAC is reset after driving > 35 MPH after discon the battery etc.
Use ECM scan tool to see ECT and other sensors output. If ECT says colder then weather low the last several hours then sensor or wires to it could have problems making engine w/ even higher idle then should.
I was only asking since this is the first car I've had that's done this. but if it's normal for a fiero then it's fine. just seems strange to me.
It's been a long time since I had a 4cyl but i remember having the same issue with high idle. Did you check the IAC valve? I'm pretty sure replacing it fixed my issue.
84 2.5L... it idles wayyyy too high. it's only when it's cold out (the colder, the higher. the coldest I've started it is around 30 degrees and it shoots up to 3k rpm.)
My first Fiero, an '87 duke, had the same issue. I did not want a cold engine racing all the way up to 3000 RPM before the engine oil even had a chance to circulate. It was also noisy as hell, and I didn't want the neighbors thinking I was revving the engine like an idiot on purpose! So I went through the trouble of installing a switch (mounted in the cab) which would cut the power going to the IAC stepper motor. I would turn the switch OFF before I turned off the warmed up engine, and left it OFF when I started the cold engine the next day. This would prevent the IAC valve from opening. Once the engine warmed up a bit in a minute or so, I would then turn the switch ON so that the IAC would function normally. This mod actually worked rather well, as long as I remembered to turn the switch OFF before I turned off the warmed up engine. It never set any codes either.
Having said all that, my '84 duke doesn't have the cold excessively high idle problem. Go figure.
[This message has been edited by Patrick (edited 11-01-2021).]
We are so use to the more modern engines, engines of today that have better sensors and programming, that can regulate an engine under all conditions. Computers on engines were a new thing back then.
We forget that old days, and how you use to let an engine warm for a while before driving.
Today, you get in it, start it up, and go!! Anything less it's a problem.
My first Fiero, an '87 duke, had the same issue. I did not want a cold engine racing all the way up to 3000 RPM before the engine oil even had a chance to circulate. It was also noisy as hell, and I didn't want the neighbors thinking I was revving the engine like an idiot on purpose! So I went through the trouble of installing a switch (mounted in the cab) which would cut the power going to the IAC stepper motor. I would turn the switch OFF before I turned off the warmed up engine, and left it OFF when I started the cold engine the next day. This would prevent the IAC valve from opening. Once the engine warmed up a bit in a minute or so, I would then turn the switch ON so that the IAC would function normally. This mod actually worked rather well, as long as I remembered to turn the switch OFF before I turned off the warmed up engine. It never set any codes either.
Having said all that, my '84 duke doesn't have the cold excessively high idle problem. Go figure.
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).]
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.
So I went through the trouble of installing a switch (mounted in the cab) which would cut the power going to the IAC stepper motor. I would turn the switch OFF before I turned off the warmed up engine, and left it OFF when I started the cold engine the next day. This would prevent the IAC valve from opening. Once the engine warmed up a bit in a minute or so, I would then turn the switch ON so that the IAC would function normally. This mod actually worked rather well, as long as I remembered to turn the switch OFF before I turned off the warmed up engine. It never set any codes either.
