Temperature issue help

[NH-503P-3]

I have a situation with my temperature gauge in my car. My car warms up fine. Fan works fine. No over heating etc. When my car is warmed up and at normal operating temp. As I drive down the road and get up to 70-80mph. On my temperature gauge the needle starts to dip down to the bottom (like when the car is off and cool bottom) not even reading bottom. When I roll at lower speeds on the streets instead of highways it comes back up. I know I don’t have any air bubbles as I have filled the coolant and run the car per proper procedures.
I’m thinking I might have a coolant sensor out. Specifically the one at the thermostat. I could be wrong.
Doesn’t the sensor going into the head work for the fan and the one at the thermostat works for the gauge?
Anybody have this issue before?
F22a6 motor also.

[sonikaccord]

The coolant temp sensor for the gauge is the one-wire sensor. The one in the thermostat and the one in the head are "switches" either on or off.

IIRC, an open or disconnected sensor should cause the gauge to go hot. I would check to make sure that it isn't shorted to the engine block or any other ground metal.

[NH-503P-3]

Thanks sonikaccord. Where is the one wire coolant temp sensor located?

[sonikaccord]

It is located under the distributor side of the cylinder head.

[NH-503P-3]

Alright I know witch one you’re talking about now. Thanks. I’m going to check that out tomorrow and trace the wire. If everything looks ok, maybe I’ll just have to replace the sensor. Wonderful I’m sure I’ll have to drain the coolant again also lol.

[Fleetw00d]

Alright I know witch one you’re talking about now. Thanks. I’m going to check that out tomorrow and trace the wire. If everything looks ok, maybe I’ll just have to replace the sensor. Wonderful I’m sure I’ll have to drain the coolant again also lol.

If you're quick, you won't loose too much coolant if you don't vent the system anywhere else. Make sure you use a new o-ring (seal) on the sensor.

[NH-503P-3]

Just so anyone else who might run into this problem knows; it was my thermostat. I replaced it then refilled my coolant & ran the car per proper procedures and no more temperature issues.