I'm going to experiment with the coolant sensor signal to dial in a "spoofed" coolant temperature.
Shown in the pic above is a 0-5K potentiometer variable resistor and a simple on/off switch. The car's coolant temperature sensor has highest resistance when cold, with values similar to this plot:
Adding my circuit in parallel with the sensor will cause the ECU see a lower resistance when the pot is turned to a lower resistance setting.
Fellow EM member
mjspiess did a similar mod (with a single value resistor) and documented the effect in his '95 Metro.
Jump forward to 0:50 to see the change in idle speed and GPH fuel consumption as he repeatedly turns his circuit on / off...