Heater suddenly blows cold, then corrects itself

[richardcox_lotus]

Evening all. I have some curious behaviour on my Sprint and looking for some guidance.

For context, a few months ago I had to send the water temp gauge off for refurbishment, which meant a partial drain of the coolant. When I got it back, coolant replenished and after a few trips the radiator cap decided to break its seal and piss water out. New cap later, coolant topped up again and after a few trips the Otter switch stopped working. New Otter switch ordered and fitted, coolant topped up again.

Note whenever I refill the coolant it’s always with the car jacked at the front, heater open, and squeezing top hose. Made sure all water up to the radiator filler neck.

So to get to the current situation. I notice the temperature gauge runs in the low 80s on quiet journeys - pops up to 90 in traffic and drops down when the electric fan kicks in.

But 2 weeks ago I included a Motorway section on my route and the temperature climbed to early 90s even on a steady stretch. However at the same time any hot air coming through the heater became stone cold. I was concerned about either an air lock; water pump issues; thermostat issues or heater valve issues. I fiddled with the heater control whilst driving and normality returned - hot air out the heater and the temperature gauge dropped (in 5 degree chunks). There was no sign of any water loss when I had arrived home.

Went out again today, and had the same behaviour - which commenced at exactly the same point on the same road ! This time I didn’t fiddle with the heater control and although the temp gauge level stabilised at about 92, there was still cold air through the heater. It stayed cold for a number of miles until I was nearly home, and lo and behold, heat was restored all by itself.

I’ve probably done about 10 trips since the last water top up so I would have thought any air would have worked it’s way out of the system through the recuperation bottle cycles by now. But is what I’m experiencing an airlock ? Or, has anyone experienced issues with the heater valve that shows the same behaviour ? I could imagine a failure of the valve that would leave it either full hot or cold, but not intermittently not working and then starting again.

Thoughts welcome.

Regards,
Richard

[tonyabacus]

Richard
Sounds like an airlock, but try without jacking the car up as this may be causing trapped air to rise to the top of the heater matrix and not clearing. Remove the radiator cap, shut off the heater valve and disconnect the heater hose at that point, lift the hose in a vertical direction and using a funnel of some sort try filling the tube with water. You can watch what is happening via the radiator filler hole, but remember to lock off the overflow tube. You can also try blowing down the tube to free any trapped air.

A further variation is to hold the heater hose below the level of the rad and heater valve and blow into the radiator, remembering to catch the water in a bucket or else wet feet. The final alternative is to try and find someone or your local friendly garage that has the kit used to pressurise the system when looking for leaks.

Some engines seem to be worse than others for this problem, then of course there are those that are no trouble at all (but I know you perhaps don.t want to hear that right now)
Best of luck
Tony

[alan.barker]

Hi Richard
I would say you need a new Heater Valve.
I think the Diaphragm in old one has crumbled.
Do you have a recuperator Bottle to the small overflow pipe next to the Rad Cap.
Also is the Rad Cap correct with 2 Valves.
Alan