The following may or may not be relevant as my car is a 2002 1.8 zetec but here goes.
I have been investigating the apparent over temperature in my car since last September (when I retired and had more time). The indicated temperature is about 97 once warmed through and up to 50 mph and on back roads at this sort of speed over the winter it will remain at that. Take it up to 60 and it will soon reach 100 and then remain around that until speeding up again and will reach 110 - ish at 70 to 75, I haven't tried higher honest m'ludd. This bothered me because once when static in traffic the fan relay failed and the engine overheated (my apologies to the two cars either side in the queue for the Eurostar who thought we were on fire). The result is quite spectacular when the expansion bottle cap lifts depositing most of the coolant over the passenger side windscreen. I used to hear the fan stop and start all the time when driving (I found a clip stuck to a fan blade, removed it and it's much quieter now) so my concern was in the event of failure of the relay at speed the contents of the radiator would be thrown all over the passenger.
I have fitted an alloy radiator (partly because the plastic one failed twice), an allow under tray between radiator and engine, moved the front number plate to inline with the bumper and fitted a scoop and now it's rare that the fan starts during normal use but the above temperatures remain unchanged.
To find out what's going on in the system I bought a 6 channel barbecue temperature sensor which blue tooths to my phone (about £30 and very good incidentally). I fitted one to the top hose, one to the return to the bottom hose and one to the bottom hose before this return and the results confounded me completely! Over the last week we went to Wales and back and covered about 650 miles, the only time the temperatures round the system increased was in traffic with little air flow over the radiator, eventually the fan would cut in when the bottom hose was at about 95 and cut out at 90 the rest of the time the top hose was about 75, return to bottom hose about 60 and bottom hose 35 to 40 so nothing like the indicated temperature. I believe the sensor is fitted to the cylinder head and it is this temperature that is being reported, I can not find a sensor anywhere for the fan so that remains a mystery but it works so if not broken don't fix.
In conclusion the cooling system seems to be working fine but I will leave the sensors there in case we ever get some higher ambient temperatures - just out of interest.
Sorry for the ramble and hi-jacking your thread. I just thought some of this might be of interest to others.
