Temperature gauge/thermostat issues

[Glofton]

The thermostat on my 98 Expedition is going directly to hot when in motion. The temperature remains normal while idling but as soon as it starts moving, it pegs straight to hot. Help!

 

[AWAR]

Is it hot enough to push out coolant? Check your grounds.

 

[99WhiteC5Coupe]

The thermostat on my 98 Expedition is going directly to hot when in motion. The temperature remains normal while idling but as soon as it starts moving, it pegs straight to hot. Help!

Are you SURE your engine coolant system is full, and does not have an air pocket?

 

[Carl Norgaard]

Yes, check for an air pocket. The genius that placed the heater core higher than the radiator cap should be taken out back and beaten. Install a t-fitting with the garden hose connection on the top heater core hose right up against the firewall which will let the air out. Otherwise you'll have to jack up the front end to get the radiator cap up high enough.

 

[GaryH]

I suppose it's possible you just have a bad temp gauge sending unit. Does it quickly go back to normal when the engine idles, or does it go back to normal slowly? If there is air in the system you should be able to burp it out by squeezing the upper radiator hose several times with the engine running.