Heater blowing cold
I'm new to this forum and I am not sure this is the right place for technical questions but here goes!
My wife's ZR 105 had a similar problem with only cold air from the heater. Upon checking, I found no coolant in the header tank - not a desirable thing on any engine, never mind a K series. I topped up the tank, but the next day after only 6 miles the tank was empty again. As a matter of routine I replaced the head gasket and water pump ( the parts are cheap and the procedure is straightforwards ) but the next day I found the tank to be empty again!
I took the car for a short drive and after a couple of miles I could hear something going on under the bonnet. The header tank was full to the cap with boiling coolant. The car, an 04 plate, was fitted with the later type of heater hose with the Saab pressure relief by-pass valve. This valve is supposed to open to allow coolant to flow along the by-pass hose to the thermostat when the heater valve is closed. I found that it had disintegrated internally and the rubber button inside had become detached and was totally blocking the inlet to the thermostat. On the K series engine, the thermostat depends on hot coolant flowing down the by-pass hose before it opens to allow the main coolant flow through the radiator. A blocked by-pass hose is a potentially disastrous event for the engine.
A replacement hose with the Saab valve is around £55 from Rimmer Brothers. I fixed my wifes car with the heater hose off an earlier Rover 25 without the valve from a breakers. He didn't charge me for it either as I bought some Rover 45 bits at the same time!
Your problem could be the same, as I understand that the Saab valve is unreliable. Drain the coolant and remove the valve. If you can poke a pencil or similar through it without any obstruction, it is likely that it has failed in the same way as my wife's car, and that the internal parts have worked their way towards the thermostat housing, possibly blocking the flow.
I hope this info is of use.