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On the 2003 with the V8 the "temp sending unit" is the "Cylinder Head Temperature Sensor". With the hood open, if you were to stand on the driver's side fender and look at the engine, the connector for it is located just above the upper left of the valve cover. The sensor has a pigtail on it. It will have two wires to it, a Yellow/Light Blue wire (reference voltage) and a Gray/Red wire (signal return).
It sounds like you're fishing for an issue. Care to share some details on the symptoms?
Rod thank you for replying so quickly. the temp guage in the cluster does not read at all so when you try to start the vehicle in morning it starts pretty normal but if the vehicle is warm it wont hardly start. A local mechanic told me that since the temp sending unit is not working that the computer automatically defaults to -40 and gives the engine to muck fuel but when it starts it has another temp sensor because it runs fine and the onboard diagnostic shows that the temp is ok. What is your thought
I scrubbed the factory service manual for the V8 because it surprised me that the PCM would use the Cylinder Head Temp (CHT) sensor as the sole coolant sensor. There's no other temp sensor showing for the engine control or the gauge. I'm used to Ford using an Engine Coolant Temperature (ECT) sensor for the PCM and a Coolant Temperature Sending (CTS) unit for the gauge. Apparently that's not the case here.
The wiring diagram for the instrument cluster shows the coolant temperature gauge as getting signal from the PCM, and the PCM getting signal from the CHT sensor so there is no other sensor that the PCM uses when the engine is cold. It would seem that maybe you have an issue with the wiring harness that allows the PCM to get a good signal from the CHT sensor when cold, but as the engine bay heats up, something expands and goes open circuit for the CHT sensor. It could be the sensor, it could be in the wiring, and I suppose it could be something else.
You wouldn't happen to have a scan tool with a datastream mode or some other OBD-II data logger (Maybe a smart phone with the free Torque application and OBD-II dongle) would you? It would be interesting to watch the coolant temperature parameter as you drive and as you attempt to start the engine when warm.
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