its a 1995 f250 which we flushed and refilled. When running the temp guage goes to the L in normal which is real close to over heating. When it comes back down it goes to the O in normal. Is this common? I was going to replace thermostat but figured I would ask here first.

 

Sounds like an air bubble in the cooling system to me. It will work itself out eventually, just keep filling up the radiator before you drive it.

 

I thought so to but we flushed it a month ago and hve done plenty of snow plowing since.

 

change the t-stat if there is no air

 

Have you checked for good connection on the heat sending unit? Or is the wire from the sending unit pinched?

 

Funny, my truck did the same thing yesterday. I just replaced the thermostat and it seems to OK. I did bleed the system for about an hour. I did have to push the upper heater hose down to get the air bubble out of it. Hopefully this is the problem. I will check the heat sensor wiring tomorrow.