Coolant temp sensor to the ECU. This would cause the car to run rich, poor gas mileage, down on power and maybe a bad idle when hot.
There is 2 in your cylinder head, next to eachother. The one with the blue connector is the ECU one, the other is for the gauge on the dash.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325