On your 91, the temp sensor on the thermostat housing sends information to the ECU all the time, not just during startup. If you have it disconnected, you'll never get out of open loop. Put the original sensor back in the thermostat housing and hook up the ECU input wire like it was originally. The stock temp gauge sensor is located on the back of the head, left side, near the firewall. You can safely remove this sensor and install your aftermarket gauge in it's place with no ill effects.