I swapped to TPI off of a 87 IROC from TBI and I can't find a fuel pressure switch for that system because the 91 had a 3 wire switch for oil pressure combined.

 

How did you do your wiring? Did you swap the tpi harness complete? Or did you splice it into your old tbi harness? I'm guessing you have a 91? We need more info on what you have and what you did so far.

 

The RS was a 91 and yes the TBI harness was modified and spliced into......probably wasn't the best way to go but...... it is starting and it will idol just very rough because the timing still has to be fine tuned and the pump hasn't been swapped out.

 

http://www.motorcityreman.com/generalmotors14.html

Cut the 3 wire connector off the TPI harness and solder on the 2 wire connector from the TBI harness
so you can use original TBI OP switch
Remaining OP gauge wire from the TPI harness goes to TBI OP sender unit

 

You described it perfect just backwards, the 91 was the TBI with the 3 wire connection at the switch and the TPI was a 87 harness with 2 wire orange and white/tan connection.