Heater Problem?

[Whysub]

Odd, but the heater on my newly acquired 1.6 8v works fine, but as soon as the temperature gauge gets to halfway, it only blows cold air. When the gauge drops a little, it blows hot again.

Checked the cooling system for leaks (none) and the oil/water for signs of water/oil, but all fine. A 200 mile round trip on motorway and stop start traffic showed no cooling faults.

Summer is coming, so it will not be a problem, but would like to know why it does this. Is it simply the thermostat?

[robiwill]

its the heater bypass valve that is blocked take it off and clean it, if that doesnt cure it then you need a replacement

[Whysub]

Thanks-at least I have a place to start the investigation.

[Organisys]

Check for airlocks.

Set heater to max hot with fan on also.
Run car from cold with coolant expansion tank cap off.
Massage all coolant hoses (including heater send) hose
Air bubbles will rise up through tank.
Wait for coolant to nearly boil.
Stop engine
Wait for coolant to cool right down,
Top up coolant if needed. Replace cap.

Worked on mine.

[diagnosis-dub]

its the heater bypass valve that is blocked take it off and clean it, if that doesnt cure it then you need a replacement

Spot on. you know it makes sense