another heater problem

[nicnet]

Hi all

Toyota Supra MkIII - 1988
White. Non-turbo.
~150k miles


I have a problem with the cabin heater.
Symptoms are:
-start car, engine warms up
-heater works fine for some time (e.g. 10 mins)
-air flow suddenly goes cool (but not cold, as in outside air cold)
-heat then comes and goes, cycling from hot to cool

also noticed:
-it often goes cool first time, when starting to drive fast (like entering motorway/freeway)
-when going from cool to hot and hot to cool, I think I can hear a vent opening/closing, inside the heating system.

The problem is not the the VSV valve- I've bypassed that as discussed elsewhere on the message board.
Although I have occasionaly heard gurgling sound in dash board, I don't think it's air trapped in heater matrix.
I have burped it a couple of times (as discussed elsewhere on the message board).
Also heat comes and goes, through the vents.
But entry/exit pipes carrying warm coolant to the matrix are both hot, so flow through the matrix seems to be good.

I think it might be one of the sensors, opening and closing a vent somewhere in the air system.
Any suggestions please?

This is quite a problem for me - this car is located in Finland, and with winter coming, it's starting to feel seriously cold.
On a related but different subject, I've read opinions saying that Supra is not a car for winter driving. I have to disagree.
Here in Finland, winter tyres are mandatory for 3 or 4 months of the year.
And with studs, I've never really had a problem, despite it being rear wheel drive.
My other car is front wheel drive Citroen Belingo (the family car - opposite end of the scale from the Supra).
It's not that much easier to drive on ice/snow then the Supra.


Nic