'98 ford taurus gauge problem

[Donnawb]

Just bought '98 taurus w/117 thousand miles.(highway) care had VERY OLD antifreeze in tank & car over heated. Coolant flush was done & gauge is going up & down constantly. It is even going to the red zone but it is not over heating. Do I have a bad thermostat? Please help!!!!

[KimMG]

Clogged cooling system? Check radiator for cold spots.
Is the fan working?
Coolant level?
Radiator cap?
What is the ambient temp? What temp thermostat are you using?
Is it overheating in traffic?

[sfontain]

Here is how I would probably approach this:

If you put in an aftermarket thermostat, then put in an OEM thermostat ASAP and do another complete coolant flush while you're at it. (You NEED to use an original thermostat from Ford; the aftermarket ones fail prematurely, if they ever work at all.) This won't hurt anyway since you said the coolant was disgusting before you changed it. Make sure you only refill with distilled water, not spring or tap water; it's less than $1 per gallon at any supermarket. Also consider buying a new radiator or degas tank (whichever you have) cap from Ford. This part and the thermostat are relatively cheap.

If that doesn't solve your problem, do make sure the fan is working. You should be able to hear/feel it turn on and off while the car is idling. And I would definitely say test your coolant temperature sensor or have it tested. The procedure is in a Haynes manual but you will need a multimeter. If it is failing then it may reading low so that the fan isn't kicking on early enough. I would especially suspect this if, during the car's constant heating/cooling cycling that you mentioned, the low end of the cycle is still reading above the middle of the temperature guage on your instrument panel.

Beyond that, sad to say you might have a clogged radiator. Usually heater cores clog in these cars, and if your heat works fine then I would be less suspicious of a radiator clog IF the radiator and heater core have never been replaced (because if the heater core HAS been replaced, it will probably be cleaner than your radiator).