1985 Engine with 1984 ECU? (Page 1/1)
BruhMans06 NOV 19, 11:14 PM
So I have an old 84 fiero 4 cylinder, and the engine was pooped so I got a new 1985 engine to replace it. So I now have the 85 engine in the car (I was already aware of the electrical differences between the 84-85 engines) but I just realized that the difference in electrical components probably means I'll need a new ECU.

So my dilemma is I have no money and I am broke, cannot buy new ECU, but I have an 1984 ECU. would I get away with using an 1984 ECU with a 1985 engine? I also have the same auto tranny that came with the original 1984 engine.

Yeah I'm dumb I know

Thanks much,


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James Bird

[This message has been edited by BruhMans06 (edited 11-19-2023).]

jelly2m8 NOV 20, 12:55 AM
put the 84 ECM in it , The only real differences is the 85 and up engines have roller lifters VS solids ( if memory serves me correct) and the Distributor is a little different, use the 84 Distributer/ Coil as those will match your harness.

[This message has been edited by jelly2m8 (edited 11-20-2023).]

Wichita NOV 20, 04:51 AM
If you need an 85 ECU, I have one you can have.

I won't charge you anything. It will be just the internal board. The case houses my EV ECU board, so I can't give that up. But I still have the original ECU board.
82-T/A [At Work] NOV 20, 11:38 AM

quote
Originally posted by BruhMans06:

So I have an old 84 fiero 4 cylinder, and the engine was pooped so I got a new 1985 engine to replace it. So I now have the 85 engine in the car (I was already aware of the electrical differences between the 84-85 engines) but I just realized that the difference in electrical components probably means I'll need a new ECU.

So my dilemma is I have no money and I am broke, cannot buy new ECU, but I have an 1984 ECU. would I get away with using an 1984 ECU with a 1985 engine? I also have the same auto tranny that came with the original 1984 engine.

Yeah I'm dumb I know

Thanks much,




Hey Bruh, I have a couple of ECMs too that I can send you. Can you let us know whether or not you have an automatic or a manual transmission? If what Wichita has doesn't fit your need... I might have something, and like Wichita, I can send it to you free of charge. Just let us know what you have... manual and automatic are different... but an 84-85 engine are basically the same except for the roller cam (which is better). As Jelly said, use the distributor from the 84.

Can you tell us what the car is doing or not doing?
BruhMans06 NOV 20, 11:58 AM
So i might just swap the distributors around if thats all it takes, Im trying to get this car finished as soon as possible. It is a 1985 engine with a 1984 auto transmission.
82-T/A [At Work] NOV 20, 12:03 PM

quote
Originally posted by BruhMans06:

So i might just swap the distributors around if thats all it takes, Im trying to get this car finished as soon as possible. It is a 1985 engine with a 1984 auto transmission.




See if that works... but let us know what's "happening" specifically... because it could just be something else that's causing whatever issues you have.

As an FYI... as part of the fire recall in the mid 80s, they actually replaced 84 engines with ~85 engines. So this was something they did regularly. It was basically to eliminate the bottom-end issue with the hairline fractures that were present in some of the connecting rods of the 84 motors.
BruhMans06 NOV 20, 12:09 PM

quote
Originally posted by 82-T/A [At Work]:
See if that works... but let us know what's "happening" specifically... because it could just be something else that's causing whatever issues you have.

As an FYI... as part of the fire recall in the mid 80s, they actually replaced 84 engines with ~85 engines. So this was something they did regularly. It was basically to eliminate the bottom-end issue with the hairline fractures that were present in some of the connecting rods of the 84 motors.



So I actually dont know whats happening yet... I just finished doing a quick rebuild and just got the motor back in the car, im bringing up the issue now because I just swapped the Wiring harnesses around. I still don't have any cables, fuel lines connected yet. I don't even have the exhaust or intake on the engine yet. Ill update once i get everything together, i just wanted to ask after I realized as I was pulling the old harness out.
Raydar NOV 21, 03:22 PM
Run the 84 ECM, and the 84 engine harness.
At the very most you might need to keep your 84 distributor, and any 84 sensors that were different from the 85. Maybe the alternator. (It might be the same, however.)
No need to "reinvent the wheel".

There are quite a few wiring differences between the 84 and 85s. I'm not sure the 85 ECM can even be used in an 84. (To be fair, I don't know that it can't, either.)

[This message has been edited by Raydar (edited 11-21-2023).]

theogre NOV 21, 05:56 PM
If installed a 85-86 Duke in 84 car & assume all external parts are Good....
Need to switch to 84 Coil & dist & use 84 wiring.
May also need the TBI and some other small parts because TBI throttle "arm" is different to fit throttle cable.

If you have to switch to 84 intake manifold... FEL-PRO MS90563

Otherwise both are same externally and about same performance even when have roller cam setup & ECM won't notice.
Many 84 duke cars have this anyway for recall/warranty replacements.

Note: If the Coil is F-d then you have to install later plug to use a new coil because No longer make coil to fit 84 plug.

------------------
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

BruhMans06 NOV 26, 07:18 PM
I started installing the wiring harness and everything has been going smoothly bu ive run into my first issue.

I bought a new cooling fan switch becauuse the original one in the 84 engine broke. I heard that fieros came with either a 2 speed switch or a 1 speed switch, i have a 2m4 with AC which I heard came with a 2 speed switch. I went online and bought the switch for an 84 but it looks wack. The female wnd is round while the male end is squared and now imconfused. in has 2 leads so im assuminhg i bought the 2 speed.



Thanks for the help!!