[Mark B]

Mark,

you might like to read this thread.

In particular the following quote by BillJ

It is the EDU, not the ECU that controls the fan. I think I might have said ECU earlier, though. So the sensor goes to the dash and the fan control comes from there.

Be aware that on cars with this arrangement, the gauge is designed to tell "white lies", just in case you should suffer any undue stress if the temperature rises under normal driving (which it will do). The gauge will stay almost rock-solid somewhere below the middle (8 o'clock on mine) under all conditions, and will only rise much above that if the system decides that, given the coolant temperature and the circumstances, there may be some cause for concern. This may be why the gauge reading bears no resemblence to the actual coolant temperature.

On our recent rolling-road session, my gauge stayed at 8 o'clock the whole time, while Gassy's earlier-style gauge quickly settled just below the red and stayed there.  [/b]

In other words, the coolant might be hotter than you think when you stop.

 

[Mark B]

One has to suspect a thermostat problem, and it's easy & cheap to replace to check.

If the engine is running hot, the thermostat may not be opening properly, only allowing a small amount of hot water to the radiator, which will be cooled quickly by the passing air. When you stop, and there is no passing air, the hotter than normal coolant will flow down the radiator until it reaches the thermoswitch, and the cooling fan will then turn on. Thus the time from stopping until the fan comes on is shorter than normal, because you do not need to wait for the whole engine and coolant to get hotter.

Hope this makes sense.

P.S. There is much debate on this topic in the thread I referenced above, as well as details of the model year changes to the cooling system controls.