Mini Cooper -- temp gauge to red zone, fan comes on
For the past year, my 2005 mini cooper with 41000 miles on it has done the same thing. I have replaced the temp sensor and the thermostat as well as the radiator cap -- had system flushed repeatedly. Wiring to sensor replaced -- STILL DOES THE SAME THING. Experimenting to see if it's a grounding problem but still can't figure it out. Has been to three different mechanics. Nobody has a clue.
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check coolant level could be too low allowing air in the cooling system causing air pocket in engine around the engine coolant sensor.coolant sensor needs to be in hot coolant so it will cause the cooling fan to come on at a set temperature.make sure coolant correct level and the coolant level in the cooling overflow jug should be at full cold mark.the cooling overflow jug should never be empty,if you see empty cooling overflow jug air got in the cooling system so make sure when engine cold every time raise hood make sure cooling overflow jug coolant level at full cold mark.if jug stay empty you have cooling leak.so replace radiator cap,if coolant level all good,i would replace the engine coolant temperature and thermostat.when your car come to a long stop or driving along long line of slow moving traffic the engine coolant blower motor should running if not have vechicle code scan for faulty coolant fan motor control module.or ecm problems.
have battery and alternator tested for free at parts store its one of them check make sure you have the correct amount of cooling fluid in radiator fans will come on on some cars after you turn them off to coll them down to proper temps ive had hondas thats done the same thing thats normal as long as your antifreeze isnt low
the fan is running because the sensor is picking up a hot spot(top of the engine(head)when you switch on again the computer is verifying the engine temp)to see if it needs choke)then shutting down the fan.....this is why one needs a computer to find out whats what with modern cars......
the fan comes on and off based on the "temperature sending unit" thats a thermal switch mounted on the radiator .. it is normal for the temperature to fluxuate as the fan turns on and off .. if it is allowed to get too hot before turning on then the temperature sending unit should be replaced ...while the car is moving the fan may never come on and isnt needed as long as air is flowing through the radiator .. as long as you are not loosing coolant then the temperatures are probably within proper range ... a sticking thermostat can also cause increased temperature range fluxuations .. that will show up as large swings of temp while driving .....
most likely is going to be a bad thermostat. when you are moving cold air comes through the grill cooling engine but when you stop you don't have that air and thermostat is not allowing flow causing overheating.
I had this exact same problem -- for about a year and a half! Replaced thermostat, replaced sensor twice, replaced overflow tank, replaced radiator cap, placed a grounding kit on the car in case the problem was electrical ... and was ABOUT to replace thermostat again and then the water pump when a new mechanic suggested we bleed air out of the cooling system. The heater core was PACKED with air, as was the upper hose valve area. Previous mechanic had been using a pressurized fill device to bleed the system and this had just been backing air into the heater core. There are two valves under the hood for bleeding air -- one on the heater core and one on the upper hose. It only takes a few minutes to fix this!