Installed a new thermostat in my 06 grand cherokee 65th anniversary and the overheat gauge goes the way to red new. Put the old one back in and its still doing it. Whats the cause? I Installed a 180 degree thermostat and the temp guage went all the way to red indicating overheating. I put the old one back in and it's still doing the same thing. I tried running the heater to get air out of it and the heater is blowing cold. My finace and i depend on this car and i need help. None of the problems happened untill i switched the thermostats.

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Raise the front of the car as high as poss with a floor jack, then fill the coolant recovery tank full and run the engine until it gets hot but not overheated, shut it off and run cold water over the radiator, this will cause the system to self bleed, do not let the recovery tank run dry. The let the engine cool down completely while still up in the front.
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