What is causing the fan to delay coming on until the car is already too hot?

If your radiator fan will not operate normally the your engine coolant will get excessively hot and your engine WILL overheat. The fan is required to boost air flow through the radiator when the temperature of the coolant gets to a specific level and prevent the coolant getting too hot..

If you run the vehicle you WILL overheat the engine without the assistance of the radiator fan and if you do that you WILL cause very expensive engine damage requiring repairs costing multiple times what it will cost to fix the current fan issue.

Either get the problem fixed immediately or do not drive the vehicle.

With the thermostat out you should see a lot of circulation. If not investigate water pump. Is the fan electric fan working? Bypass the relay and run the fan whenever the engine is running. If it doesn't overheat with the fan running then investigate fan control. make sure the coolant is topped off and use a thermometer in the radiator to verify temperature.

The causes of overheating are:
1. Wrong mixture of water and antifreeze.
2. Clogged radiator inside or outside
3. Radiator cap leak.
4. Bad water pump.
5. Thermostat wrong, or installed upside down.
6. Bad water pump. Vanes on impellor worn out.
7. Hose collapse on the intake side of the pump.

sorry would need my head nder the bonnet to know the cause ,if Ac works whilst driving then suspect a fan not coming on or a piece of plastic in front of the rads ,but this is outragous but yes i have seen this ,and dead flies and bugs blocking a rad but i would really need to see a vehicle with this sort of problem to know the cause

Have the cooling system pressure tested with a special tool and see if the source of the leak is the water pump, also feel under the pump for antifreeze.

For car running hot and overheating issues, I suggest to you go through this help links. Click this link below and go through the troubleshooting procedure to take care of the problems: ----

temp gauge goes erratic? http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html

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Temperature gauge keeps fluctuating up and down close to red? http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html

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Car runs hot? http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
Car freeze overnight and started overheating while driving?http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html-----------Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------overheating? http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------Car engine Overheats and transmission starts slipping?http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
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Temperature gauge does not move and no heat when heater is ON? http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

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This details will help.Thanks.

Bad thermostat

if the fan electric fan is not comining on, you need to check the fan motor to see if your motor is bad on the fan. check it by running a hot wire directly from your car battery to the fan elecrtic plug.

Check the thermostat. If the upper hose does not feel very hot to the touch when the car is warmed up already, this indicates the thermostat is not opening, and therefore the water cannot circulate causing the overheating problem. Second possiblity is the radiator fan. If the car tends to overheat only on city driving it is a the radiator fan not coming on. This is either due to a bad radiator fan, blown fuse,bad relay, or a bad temperature sensor or temperature switch. If the car tends to overheat at highway speed, then it is the radiator being clogged up, and not letting enough water circulation when demand is at its highest which is high engine rpm.

try washingthe raditor at a car wash wash thre the engine compartment to the outside. clean the ac fan condensor then check that the fan is not loose next replace the anitfreeze if it's more than two years old, and the radiator cap if it still does it replace the thermostate gasket, these are the most common problems for over heating. you could have a clutch fan bad too but do the other stuff first it's cheeper.