1988 Mustang Lx Surging problem

I have installed a autometer water temp gauge cause the stock unit went out in my 88 LX. I put the sending unit thats autometer supplied in the stock place and just dissconnected the original sending unit from the harness. when i got it all intalled i started the car and you could tell it was running rich and at an idle it was surging between 4oo to 1000 rpm. So I Plugged the stock unit back in and it stopped. How Do I get rid of the stock sending unit without having the surging problem.. Thanks