So I have a 1989 s13 with a blacktop sr20det, let a buddy borrow my ecu to test something and he fried it. Ordered me a new/used one and I plugged it in and it immediately had a misfire on cyl 3, idle was terrible so I swapped it with a good coil pack, nothing changed, swapped a new injector, nothing. Tried resetting it and it went back to normal! Thank God. Until today I was pulling out and all the sudden the car started to misfire and run extremely lean.
Moral of the story:
3 codes flashing on my ecu
11-cam angle sensor
12-maf
34- knock sensor
Tested knock sensor and it was bad, changed it with a new one and it is still doing the same thing, swapped maf with a know good one and nothing, pulled out cam angle sensor to inspect everything looked fine.. I tried swapping the ecu with my brother to see if his worked and the car ran fine..
So it's got to be the ecu right? But why would it work fine and then all of the sudden not?

It may have to heat up really good to show the fault..................

What may have to heat up?