My 2000 Jaguar S-Type High Engine Temperature message comes on and the temperature gauge goes to maximum hot for about 45 seconds then goes back to normal within a few seconds. This happened about 4 times in a 35 minute ride.

Remove the gauge cluster and check for a bad ground. If it is bad you can resolder the trace on the back of the cluster back to the groung pin for the connector.

One advantage us dealer tech's have over a week end warrior is that we have factory service repair information and have factory scan tool ,Tech 2 , to check data coming into the instrument cluster . Most of you probably don't even think about how that coolant temp infor shows up on the gauge , except by a coolant sensor !

High Temperature Indicator
The high temperature Indicator function handles the control of the Engine Temperature Gauge Display and the red High Temperature indicator. The Powertrain Control Module (PCM) reads the Coolant Temperature Sensor, filters the messages, and sends the coolant temperature to the instrument cluster via a Class 2 serial data message. The PCM monitors the coolant sensor in order to determine a fault. The PCM transmits the Class 2 serial data coolant temperature report messages to the instrument cluster for the temperature gauge display and for the high temperature indicator control.
The instrument cluster receives the engine temperature data in the Class 2 serial data message from the PCM. The instrument cluster converts this data into a temperature gauge pointer position which drives the gauge pointer. The instrument cluster also uses this data in order to determine the high temperature indicator control.
The Body Control Module (BCM) looks for the high temperature indicator Class 2 serial data report from the instrument cluster and triggers a chime when the indicator is ON.

Your best bet , take it to a qualified repair shop , rather then just replacing parts .

Jeff, you haven't said what the problem is or what vehicle you have.

However, be aware that some vehicles have two coolant sensors.

One sensor sends a signal to the ECU to help control the running of the engine. The other sensor sends a signal to the temperature gauge on your dashboard - it shows you what temperature the engine is at - i.e cold, normal or hot.

If your vehicle has two coolant sensors, failure of the first type would more than likely illuminate an engine check light.

Failure of the second type would cause the dash temperature gauge to permanently read cold or hot, irrespective of the true engine temperature.

To locate this second type of sensor, look in the area of the thermostat housing. It screws into the engine block and will have 1 or 2 wires connected to it.

The link below shows generic images of this second type of sensor:

coolant temperature sensor Google Search

The gauge is supposed to move. Between 190 and 210 degrees. About a 1/16 to 1/4 inch movement with the needle. If it goes from middle(200 degrees) to hot(260 degrees) and back. Check the coolant level. Air pocket in the engine. Low fans come on at about 210 degrees. High fans come on at 225 degrees. High fans come on when the A/C is turned on. And it works.

I'd say you have a faulty thermostat, be it new or not. If your temperature gauge goes to cold as your engine is taking you down the highway, the thermostat is not doing its job. The temperature should never come down to below normal until engine is shut off...

Watch the temp gauge when the electric radiator fans come on. The temp sensor that controls when the fan comes on may be faulty.

Your description indicates the engine/coolant thermostat is faulty and allowing at least some coolant to continue circulating so at higher road speeds the engine is over cooled.

I would suggest changing the temperature sending unit on the engine.

It would not be abnormal, so long as the fan does not come on when you switch on from cold. The fan's job is to draw air through the radiator to cool the contained liquid to a suitable working temperature. The fans sensor is activated by the temperature of the coolant. If this is a problem which worries you, try driving as gently and slowly as possible for the last mile or two of your journey. If things are normal the fan should not run on switch-off as that 'trick' will have allowed the coolant to get below the fan-kick-in temperature before you stop the engine. If your vehicle has a temperature gauge use the rteading as an indication.

http://wiki.answers.com/Q/Should_the_radiator_fan_run_after_the_engine_is_turned_off

The 12-cylinder engine is a hot-running engine to begin with, so if you've changed thermostats, did you install one that opens at a lower temperature? (that depends on your local climate too, but your thermostat may be stuck-closed, or opening at a higher temp than desired).

does your car have an Auxiliary Electric Fan or two?,(in front of the radiator?), Is it coming ON when the gauge reaches a certain temperature? If not, the auxiliary fan Relay, Fuse, or Thermo-Switch (if applicable) may be bad. I am guessing here because I am not familiar with the particular model, but the 12-cylinder engine since the 1970's has a pair of electric "air-pusher" Fans which came wired from the factory to come ON and stay ON as long as the A/C compressor is ON. In other words, they should run as soon as you turn theclimate control to ON. Compare that to the XJ-6 which has a Thermo-Switch to control the temperature at which the Auxiliary Fan comes ON.

All the above assumes your Cylinder Head Gasket is good, and that you have the required mix and quantity of coolant & water in the system. A product called "Water-Wetter" might help with lowering the temp, you just add it to the coolant/water mix. It is usedin race cars. Whenever topping up the coolant, start with a COLDengine, turn the HeaterON to maximum temperature so thatthe Heater Valve opens when the engine reaches operating temp and allows coolant to fill the Heater system too.

And last: is the Water Pump good? No leaking through the "weep hole" ?