Car suddenly overheats, then cools
You need to install a new thermostat. The stat is located at the lower end of the top radiator hose within the water outlet. After the stat is installed, run the car with the radiator cap off for about 20 minutes checking and adding coolant as the air is pushed out of the system. Keep a eye on the coolant level over the next few weeks to insure the system does not have a leak which could be why you had this problem in the first place. The
thermostat is more than likely stuck shut from the overheat. Have to be changed.
There are a few possibilities as to the cause, bad thermostat, low coolant level, inefficient water pump, restricted / clogged radiator, and so on... The reason it cools down, is the heater unit in the passenger compartment is actually a small radiator, and when you warm up the interior, you are cooling the coolant, so to speak. Also, a lot of the newer vehicles will run the electric cooling fans when the heater / defroster is on, and this will cool the car down if it is overheating due to the cooling fans not being turned "on" by the coolant temp switch. To check if it the fan circuit, let it idle in your driveway, and when the temp starts climbing, the electric fan(s) should turn on when the coolant temp is about 225 deg. farenheit. If they don't turn on then, put the heater control to defrost, and then, if the fan(s) come on, you know where to start looking...
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1997 Jaguar XK8
on Sep 28, 2009