so I got the fiero on the road and it runs really well.. when cold.

I have an iron duke that runs well when cold, but once it hits operating temp it starts to bog down and stalls quite often. I heard it could be an o2 sensor thing but idk.
also when it starts it will slowly climb to a little over 2k, but it keeps climbing after that, ive just never let it go past 2k and after a while it will very quickly drop to around 700 rpm and sputter a little bit (this happens only when warm)

another issue is Ive bled my brakes but they still squishy, the master cylinder looks newish so i doubt thats the issue, I might just need to bleed them a few more times

help pls

thanks much


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James Bird

Certified Idiot

[This message has been edited by BruhMans06 (edited 03-28-2024).]

year of car? it maters.

Brake problems see https://web.archive.org/web...cast.net/~fierocave/ brakes & rear brakes.

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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

it is a 1984 but the engine was blown up so I swapped it out for an 85 engine but still retained the original 1984 electronics.

If the problem doesn't start until its warmed up, then it is likely not the 02 sensor.
The O2 while isn't used when the car first starts up, it will kick into the closed loop and take the 02 input before that. Maybe around 120F.

Probably gonna be hard to diagnose if you don't have a check engine light with codes. Really need a scanner/logger so you can see if all the sensors are reading normally.

It could always be the ECU (computer). These are now 40 years old, and they don't last forever. I had one that would cause the car to die but only when the car was hot. When I started up cold it ran great. Changed the computer and problem went away. Not saying that is your problem, but something to keep in mind.

The O2 sensor is running often in seconds to a minute when it reaches 600°F as exhaust heats it that fast.

O2 sensor is likely Not your problem.

Can be a lot of other things, often several @ same time.
Start cleaning/fixing every "ground" bolted/screwed to engine & body. "lube" them w/ silicon or pematex green label brake grease to keep out "water."

see https://web.archive.org/web...cast.net/~fierocave/ wire service

[This message has been edited by theogre (edited 03-29-2024).]

Ignition and pickup coils and ignition modules can break down with heat and cause an intermittent break in the ignition circuit. That can cause a backfire. I doubt it's fuel related as that isn't a sudden on and off condition. It could also be a loose connection in the ignition circuit. Maybe a wire that's in an airflow stream.

I doubt its the ignition coil since i just replaced it a few weeks ago. Ill double check my grounds, i did end up dumping coolant literally all over the engine on accident but I dont know if that would affect anything. If the ECU really is the issue, then I guess Ill be starting the engine swap a little earelier then anticipated.

quote Originally posted by BruhMans06: I doubt its the ignition coil since i just replaced it a few weeks ago. Ill double check my grounds, i did end up dumping coolant literally all over the engine on accident but I dont know if that would affect anything. If the ECU really is the issue, then I guess Ill be starting the engine swap a little earelier then anticipated.

Coolant, mixed or straight, is Electrically Conductive & will "soft short" many connectors, Tack Filter, & more.
Anything making a Soft short will carry enough volts/current/both to make big problems but usually won't blow fuses or cause a fire.

You may have to Replace parts including the Coil as Coolant gets in easy then don't get out or if "dries" now, just waiting for next wet day to have same headache. Can ruin spark plug wires too.

Think bad for 85 Duke? 87+ Duke w/ DIS has 2 Coils & ICM below the Intake & several spots like to leak coolant on them.

84 Fiero's had like 8 Eprom updates for erratic / high idle issues. Once you make sure you don't have a gasket, vacuum line, EGR issue, you might want to dig for Eprom calibrations. It's been many a year for me, it escapes me what the last Eprom number is.

What model year is your ECM? only real difference between 84 and 85 is a roller camshaft which the ECM doesn't care about.

[This message has been edited by jelly2m8 (edited 03-31-2024).]

thanks yall, I guess my next course of action is just checking over all my grounds and drying the engine off of any residule coolant and such, ill give an update in the nmext few days whether it fixes the issue or not.

quote Originally posted by BruhMans06: ...the coil was dry and Im getting spark to all of the plugs. now the engine doesnt want to run at all.

Had you timed the ignition, with the ALDL jumper in place?

Before you give up on this engine, see if you've got fuel pressure... and/or spray starting fluid into the throttle body.

[This message has been edited by Patrick (edited 03-31-2024).]

I think Ive either lost compression or fuel pressure, so ill check on those onec im ale to buy the tools to test

[This message has been edited by BruhMans06 (edited 04-01-2024).]

My money is on the coil. I had an 88 act exactly the same with a new coil. Turns out there was a hairline fracture that opened when it was hot and grounded to a mounting bolt. It still sent spark, so it was very hard to hunt down. Saw it ark intermittently in the dark one night. New coil number 2 fixed it.