I bought a generic OBD2a to OBD1 conversion harness and I when I plug it into the OBD1 P72 I have and start it, it runs fine without any CEL but there is a loud hissing noise like a gas leak. When I plug the OBD2 P72 back in, it returns to normal. My question is: is there anything I need to do to the conversion harness to make it compatible with the P72 and what would that noise be?

The noise sounds like it is coming from underneath the car in the engine bay. I want to get this working so that the ECU is ready to go in with uberdata for tuning.