If it's idling at 1500 rpm once the engine is warmed up, that is too fast. 800 is typical.
There's two coolant temperature sensors. A single-wire sensor is used for the dash gauge. A two-wire sensor is used for the Engine Computer. Both types have an extremely low failure rate because they have just one component inside them. However, in the early '90s, Ford had a lot of trouble with their two-wire coolant temperature sensors. The most common problem was an erratic reading. The Engine Computer increases idle speed at colder temperatures, and the erratic readings resulted in erratic idle speed. A less-common symptom was the sensor read a temperature colder than the actual temperature, but the readings were steady. The result was an idle speed that was too high.
The way to find this is to connect a scanner so you can view live data and see what the computer is responding to. If the dash gauge reads normal, and you get nice hot air from the heater, you know the engine is reaching normal temperature. If the coolant temperature sensor shows 140 degrees, for example, suspect the sensor is not reading properly.
If you find the temperature displayed is too low, and the air from heater is cool, suspect the thermostat is stuck open and the engine really isn't getting hot enough. That will keep idle speed too high.
Thursday, March 2nd, 2017 AT 4:53 PM