Heater Control Valves - ka fiesta

[pistnbust]

Go easy cos its my first post

Its apparent from looking on the interweb that a faulty HCV is a major cause of the engine overheating , the kids fiesta has just started to run very hot so im just about to change it, The car is giving all the symptons of the Thermostat being stuck closed, but going to change the valve before the thermostat as its far easier

Im really interested in why it does cause overheating
i would have thought the HCV either sends water through the heater matrix or not in varying amounts, whats the difference between a faulty valve and one that works correctly that would cause the engine to run hot

[Karlos TDCi]

When the wife's Fiesta needed and HCV change, it had no bearing on the rest of the coolant system or water temp. I have heard that overheating water temps are sometimes related, but we didn't experience that.

[over_the_hill]

If the thermostat is failing and the temp is hot that might affect the HCV by sending water that is too hot through it.

But the HCV should have no affect on the overall cooling system. It's a straight in/out to the matrix that is tapped off the main cooling circuit. Also what about when it is warm and the HCV valve is not being used - no blower or set to cold.

A simple test is to let the car run for a while and then feel the top and bottom hose on the radiator. Once the thermostat opens one should be hot and the other should be warm as hot water flows in and cooler water flows out. If they both stay cold or cool then it's probably the thermostat staying shut or not opening properly.

[pistnbust]

Thanks for the replies

I changed the HCV a couple of days ago and all seems OK
Temperature is back to normal, I little lower than before it started overheat if anything
I took the old valve apart and found 3 thin bits of blue silicone sealant inside it, maybe these were causing the problem

This supports everything I read on the internet that a faulty HCV can cause overheating, but I still cannot see why
I asked the Wife and all I got was "BECAUSE IT DOES"

I did see somewhere its possible that the HCV valve supplies hot water to the thermostat, heater hose definitely goes that way, maybe it can fail in a way that restricts the flow to the thermostat causing it not to open, my heater appeared to work with the old valve, in that it blew hot and cold. With the new valve the heater seems to react differently to the Temp knob in the car so although it appeared to work maybe it wasn't working correctly and those bits of silicone causing something weird to happen,
Still would like a definitive answer as to what's going on

Incidentally I was surprised how efficient these cooling systems are, my first conclusion was that the thermostat was stuck as the top hose got hot and the bottom didn't, after changing the HCV the situation didn't change that much, top hose hot bottom hose coolish, I checked it with the car sitting on the drive ticking over and temp gauge at normal, Wasn't until I took it for a good 15 min drive the bottom hose started to warm up, Even with this bloody cold weather it never got as hot as I expected,
My old crossflow never cooled as good