QUOTE(SKYjack @ Jan 15 2016, 10:33 AM)
There is a continuous input from the temperature sensor to the ECU, throughout engine operation. ECU uses this input to command appropriate fuel strategy. Hence it is critical the temperature has to be 90 deg, not less not more. 90 deg is the optimum operating temperature.
Air density is higher during cold mornings. This would require a slightly richer fuel ratio. Due to improper temperature inputs the ECU has not been able to adjust the right fuel ratio. Therefore sluggish or difficult start up.
Your over boil coolant temperature is also due to wrong feedback signal from coolant temp. sensor. Every time a 70deg temp. signal feedback is received, ECU commands the engine to increase temp to 90deg. Continuous high temp. commands leads to over boil condition.
Coolant Temp sensor the one which provide signal for ECU to activate/control fan speed, dashboard gauge and also input data fuel ratio.
However, coolant temperature is control by thermostat and temperature sensor.
thermostat will stay close to heat up the engine and open @ 89deg to allow coolant to flow thru radiator for cooling.
If coolant temperature sensor is faulty, it cant provide accurate signal to ECU to activate fan in time, causing engine to heat up.