I had to send out my ecu to get reprogrammed today so I borrowed my friends ecu. It came out of his 93 SE-R. My car is a 91. I figured it should just plug and play with no issues considering both are OBD1. Is there some difference in them? My check engine light now comes on when I apply like 1/2 throttle or so and flickers on and off. Only did this since I swapped ecu's. Will I be safe to run this ecu for a couple days until my ecu comes back? The light does worry me a little. TIA.

flickering...make sure you have the ecu firmly plugged in and evenly seated

I'll take a look at it tomorrow. I did throw it on quickly because I was in a rush. I figured maybe the ecu was a little different because I know the 91-92 harness is different from the 93.

Yes, should be fine - my '93 even ran with no codes using a '94 lowport. Be VERY nice to that monster connector seating it to the ecu, and make sure you have it in there straight and all the way in. If it still throws code, may be the ecu. Long as you have oil pressure, you're not detonating, and it's not overheating, you should be okay.
Jer