Engine Temp sensor

[mexicana]

i have a 91 hardbody KA24E. Its been getting horrible gas millege so i used the bult in diag light and got a code for the engine temp sensor. Tested to see if it works and its within speck so checked to see if it had voltage it had 4.88 volts that was good. So i pulled back the boot and back probed it with it connected with the sensor heres the wiered part it goes fwom 4.88 on the power side to 0.0 when connect? thought i had a short nope bypassed the whole thing with 2 wires ran next to harrnes and still the same thing. It has to be something with the ecu but what? And know the it pulses when its ran at 2000 RPM. Please help!!!!!!!!