Found this that may lead to Gilbertds’ thoughts….
Engine Coolant Temperature (ECT) sensor - (from99MY)
The ECT sensor is located at the top front of the engine, adjacent to the coolant outlet pipe. The sensor screws into a thread in the inlet manifold and incorporates a sealing ring between the faces of the sensor and manifold.The ECT sensor multiplug has four wires; two are thesignal and ground connections used by the ECM, the other two are used by the body control module(BeCM) for control of the temperature warning lamp operation on the instrument pack.The sensor contains two thermistors with negative temperature co-efficients; as temperature increases,the thermistor’s resistance decreases. The ECM receives a corresponding analogue input voltagebetween 0 and 5V.NOTE: The temperature / resistance characteristics of the two thermistorsdiffer, and so it is important to maintainthe correct pin-outs.The ECM uses the information received from the ECTsensor to make adjustments to the engine operatingconditions. The ECM ensures a richer air:fuel mixtureis available at lower block temperatures for good quality starts and smooth running. The mixture is then made leaner as the engine temperature rises to maintain low emissions and good performance.For NAS vehicles with secondary air injection, thesignal from the ECT sensor is monitored at engine start, to determine whether the conditions are coldenough to warrant secondary air injection to beemployed. The ECT sensor is then monitored to switch off the secondary air injection when therequired engine coolant temperature has been attained.
If the sensor fails, the ECM uses a substitute software routine that changes default value during warm up,based on the signal from the inlet air temperature sensor. When the software model reaches a coolant temperature of 60°C (140°F) the ECM implements a fixed default value of 85°C (185°F). The ECM coolant model also forms part of the diagnostics that is performed for detecting a temperature sensor fault, as well as open and short circuit tests.
NOTE: All voltages listed are approximate.
A coolant temperature circuit failure may result in the following symptoms:
• Poor cold and warm/hot starting and driveability.
• Instrument pack temperature warning lamp willilluminate.
• MIL will be illuminated.
• Temperature gauge reads excessively hot orcold.
• Cooling fan will not run
• SAI pump will operate at engine start up even when engine is hot (NAS with secondary airinjection system only).
I will try change the temp sensor.