KA sport overheating! How to test heater control valve?

Hi all, first post..

After reading some of the posts about KA overheating probs it seems that a lot of the times its down to the "heater control valve" .

Is there anyway I can test to confirm before I replace it?
Bypass? Block off? etc..

Symptoms:

Heater works ok.. Pipes hot..
Overheats but only after several miles..
Rad fan kicks in BUT rad doesnt feel hot ! just warm..
Expansion tank cold/warm..
No pressure in system..
No water loss except if you ignore temp then just boils over..

There is no pressure in the system even when the overheat light comes on.. I dont understand how there is no pressure even when normal temp could it be expansion tank cap?

Pressure tested and all ok..

My thinking was initially thermostat but can see a lot of heater control valve related overheats..

Opinions and test suggestions very welcome.. TIA Kev..

I'm sure it will be the thermostat, the heater valve only stops the water running through the matrix not the radiator.

No pressure, unless you have a leak, will be the filler cap.

These heater valves are prone to leaking, and loss of coolant initially gives you overheating and eventually a blown head gasket.....best to keep an eye on the level.
I'm sure Peter is correct, but when the fan is running, the radiator core should be hot to touch, all over.
John

Thanks both for the replies..

Yesterday out of curiosity I thought I`d try something .
Without having to top system up from overheating the day before..Levels ok etc..
I turned the heater to cold setting and drove it 26 miles and there were no probs! and the radiator was hot this time (I mean as normal not overheat) showing to me that the thermo had opened with the heater on cold.

With heater on I can only drive it about 3 miles before the overheat light would show and then the radiator was only warmish..thermo shut?

There is no water loss unless it is very small and even if the heater valve was leaking turning it to cold only diverts the water away from the matrix and doesnt stop the flow.

I have been nosing around a few sites and the heater valve does seem to be a common cause of overheating on the KA not it leaking but somehow failing and then affecting the thermo. Strange.. scratches head..

IF it is the heater valve my theory is:
Both heater pipes in/out are hot with the cold setting and on the hot setting the return is cooler (obviously being cooled by heater rad) this return sits nearly directly on top of the thermostat and I think that this cooler return is preventing the thermo opening.

I`m trying to think this through logically
Heater on = thermo doesn`t open (cool rad) and then overheats..
Heater off = thermo open? (hot rad) no problems..

Thanks for the suggestions they are what I initially thought the problem would be they all make common sense but still cant really understand how the heater on/off opens the thermo.

Either way this doesnt explain the loss of pressure and I will sort that first, as you suggest prime suspect is the cap as there doesnt appear to be a leak..
Thanks again and I`ll post if I find the prob..

Its somewhat bizarre but I would rule the thermostat out by replacing it....perhaps it is sluggish to open, and is only fully open when the temperature is very hot....clearly it is doing something as the engine would have boiled over on your heater off run if it was stuck shut.
John

The only way I can see that the heater being on would cause overheating is if the pressure in the cooling system is very low, due to perhaps a faulty water pump and the water is flowing through the heater instead of the radiator, in which case the heater should be very hot, very unlikely I would have thought. The only other suggestion is that the radiator is clogged. How many miles has it done?

Certainly with all of the vehicles I've worked on, if the cooling fan is running, the rad is as hot as hell and the hoses well taut under pressure, as you would expect.
I don't know where the temp sensor is on the Ka - lots of cars have the fans controlled by the ECU these days.
John