I've posted this elsewhere (on a more "specialist" forum) but without any sensible answers, so it's worth a try here... I doubt any of you will have come across this prob but you may be able to suggest a sensible way forward.
This (my daughter's, (previuosly SWMBOs) car has been with my mechanic a number of times for this problem but we've not yet fixed it. it looks remarkably like problems reported on older Fiestas caused by a faulty heater control valve, but I'm not sure my car has one - certainly can't see one in the engine compartment, as per YouTubes...
So the symptoms. Firstly, the car will sit idling for hours without overheating, The fan kicks in correctly.
My mech plugged in his device today so I could drive for a bit and test extensively. In short:
It is definitely overheating - warning light comes on at 120 degrees. The car first went in because of steam escaping, so it isn't just a false reading.
If you then turn on the cabin heating it drops fairly quickly back to about 102 (this is on motorway driving) and stays around there, or a bit higher.
Mostly the bottom radiator hose does not get even warm during/after driving, but after one test today I left it idling with heater and blower on full for about 10 minutes (temp about 100) - temp dropped to 90.and both rad fans were pretty hot.
Both of the heater pipes were hot each time I checked them.
The temp change when turning off/on the heating when running at 70 mph is entirely reproducible.
The car has been in the family since it was 6 months old and it has never had any parts replaced or gunk added to the coolant. It has about 45k miles and FSH, mostly Ford. It used to be the wife's car so no-one will have driven it hard.
So far it's had a new radiator cap and fan (proven not working by an automotive electrical engineer!). The rad has been bled and the mech used a sniffer in the coolant tank and said there was no evidence of head damage. The coolant levels aren't going down. He advises that the pump appears to be working, the rad doesn't appear to be blocked and the hoses all seem OK.
So we are stuck. Having read other threads I've asked him to replace the heater control valve but as above, I'm not sure it has one (though I feel it must).
If that doesn't work I guess it will be a water pump then a radiator, but there is nothing else we can think of. I am trying to limit the cost too!!!
Any thoughts would be much appreciated. Either about HCVs, what to do next, or other diagnostics/tests I can do to narrow it down.
Only suggestion from elsewhere is to try taking out the thermostat and see what happens, but I'm not sure what the expected outcome would be and what it would prove.
I don't want to influence any thinking but it seem to me that the stat may not be opening. Therefore it needs a bit of thought around why this might be, if the new stat isn't duff. My pwn theory is still around the thermostat. Apparently the hot water reaches the stat from the heater outlet which is why the heater control valve is a possibility. But although I imagine there MUST be one, it is definitely not in the same easy-to-access lalce as the older Fiestas.
I'm adding this at the bottom as it may be a red herring but the AC doesn't seem to be working that well either, with vaguely warm air from the vents. I'm thinking this could be consistent with a problem in the heater area but not really sure what.
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