temperature gauge goes haywire after replacing CTS

[MGiovanni]

I just replaced the Coolant Temperature Sensor after the service engine soon light lit up in my 2003 Moutaineer. The analyzer showed the Coolant Temp Sensor Circuit to be faulty so I replaced it. Now the gauge goes haywire from low temp all the way up and back. Could I have installed a defective sensor or should I wait longer to change it as there may be air trapped in the sytem at the sensor? I've been driving my Mountaineer for about 1/2 hour so far.

 

[gtyates]

Air in the system. You need to "burp" the system. Basically park it with the front end slightly elevated. With a cold engine, remove radiator cap (or the cap to the reservoir tank). start truck, turn on heat with temp to the hot side, and let it run for a bit. If you watch you will see air bubbles actually rise and pop. I had to this to my daughter's Mazda 626 after I replaced the reservoir tank.

 

[MGiovanni]

The temperature sensor WAS defective. I went in the house for a cup of coffee and to gather my composure and make a diagnostic flow-chart. When I came out and tried to start the vehicle, It would not start. The temperature guage needle pointed all the way to the top indicating a severe overheating situation. I tested the Sending Unitwith a multitester and found that it had grounded out fully. The vehicle was sitting in the garage and had been off for about an hour and the engine was cool. I discovered that when the sending unit causes the gauge to "top-out," the fuel pump shuts off preventing the vehicle from starting. Went to Advance Autoparts and replaced the sensor with a better one. Problem solved. Ten minute fix. Good idea to change the thermostat as well. My daughter's mountaineer stays nice and toasty in these crappy Chicago winters.