i have an 85 fiero 4 cylinder with a 5 speed and the motor is shot. i just got my hands on an 84 fiero 4 cylinder. i was looking at the motor and there are some slight differences with the distributor and the wiring harness looks different. i also read that the 85 motor has roller lifters.

my question can i just take the engine harness and distributor off the 85 motor and install it on an 84 motor? then drop in the car? wasn't sure if the computer programmed for an 85 would cause an issue.

You change change 85 parts to 84 engine (may need 85 TBI and other parts so throttle cable works etc.) but...
Is a true 84 engine that often have one to several issues even if seems in "good shape" now.
84 and early cars may had engine replace under warranty/recall and that engine likely Isn't 84 long block but 85-86 long block.

See my Cave, Do You Recall? & Duke Quick Ref

What's wrong w/ 85 engine?
Some problems are easier to fix then replace the engine.

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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

the 85 engine has to be rebuilt. you can hear the grinding noise and i found metal shavings in the oil pan. i may keep it and get it rebuilt later. just wanted to get it running on a low budget right now. i did have to change out the throttle plate for the cable but everything else looks good. i will install it and report back how it goes in case someone else is in the same situation.