A good test for the HCV on the older cars was to set the heater to hot (assuming the valve works). If the overheating problem improved or went away, that was your culprit.
We had this issue on a mk4 Fiesta. We bought it on a cold, snowy day so the heater was on max, and it was OK. As soon as the weather warmed up, it started to overheat. I didn't make the link initially with the heater setting, but rather found it through trial and error. But not before I'd replaced the thermostat and flushed the radiator, neither of which made any difference at all.
It would idle all day long without overheating, regardless of heater setting. When driven, it would overheat within a couple of miles. Unless the heater was set to full hot, in which case you could drive it all day without an issue.
We concluded that there was some sort of fault or internal blockage inside the HCV that wasn't allowing sufficient coolant to bypass the valve when it was closed off. When the valve was open, coolant flowed normally.
I put a new (genuine Ford) HCV on it, and the problem went away. A huge relief at the time.
Last edited by: DP on Fri 28 Jun 19 at 14:15
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