Air lock in cooling system. bleed valve? getting water in to heating matrix?

sounds like your heater matrix could be blocked try flushing out but when you refill system undo one hose on your matrix at the rear of engine compartment to bleed air out of matrix this should help if not it may be you need a new water pump

Possible trapped in system. Bleed system, should have two small screw bleeders, one in front of left valve cover on pipe behind water pump and the other on coolant pipe outlet as you follow upper rad. hose towards engine. Engine cool, remove rad cap, open both bleed screws and fill rad until coolant flows free out both bleed ports. close both bleed screws, rad cap off, start engine and let idle until upper hose is hot to touch , shut off, let it cool down for 20 minutes, top up rad and install cap. restart car ,drive and check.

You may have air in the cooling system, the air bubble or gas bubble in the case of the head gasket leaking creates a block to circulation of the coolant. Google how to bleed the cooing system

First try a air bleed to the heater matrix as these are prone to getting air locked after cooling system work or low water conditions. you can separate the hose and gently push water through with a garden hose, this is safer than trying to pressure bleed when the engine is hot. Other potential causes are twin solenoid water control valve (on this car I think its fitted RHS by lower front radiator, check wiring to it as there have been issues with corrosion and bad contacts in the connector. If all of this is okay you will then have to check voltages to the valves and the correct action of the cabin temp control unit - this bit is best left to a garage unless you are competent with a DVM. good luck.

Is this a quantity of air problem or quality of the heat from the heater?
If its heat from the heater you need to first flysh and reverse flush your cooling system, includes radiator etc and use a good flushing agent first. Then replace heater water valve.
Refill and bleed out cooling system and get car up to temp.
When hot, feel pipes going into and out of heater matrix. On full heat they should both be roughly same temp, if not , replace matrix cos it must have collapsed.
Have yoy EML light checked at local garage using spec diagnostic tools.
Roger

normaly bleeds itselfs. fill up coolantreservoir to the max and start engine. open heating max. now hoses going inside must get real warm. and heat must go inside. temp gauge must go to middle if stays cold, check thermostatic valve. good luck

SOUNDS LIKE YOU MAY HAVE AIR BLOCK, OR BLOCKED HEATER MATRIX, CHECK FIRST CHECK THE HEIGHT OF THE HEATER MATRIX IN THE CAB , IF ITS HIGHER THAN THE FILLER/EXPANSION TANK. THEN COULD BE AN AIR LOCK, LOOK FOR THE HIGHEST PIPE AND SEE IF THERE IS A BLEED VALVE, IF NOT TAKE BOTH PIPES OFF THE MATRIX FEEDS (TWO PIPES GOING FROM ENGINE BAY TO INSIDE OF CAR, AND FLUSH OUT WITH HOSE TO SEE IF IT IS BLOCKED, AND RECONNECT LOWEST PIPE FIRST AND FILL SYSTYM TILL WATER APPEAR FROM TOP AND RECONNECT RUN MOTOR, AND KEEP TOPIN UP, YOU CAN BY A COOLENT FLUSH SOLUTION TO CLEAN THE SYSTYM OUT

CHECK THE HIGHT OF THE HEATER MATRIX IN THE CAB , IF ITS HIGHER THAN THE FILLER/EXPANSION TANK. THEN ITS AN AIR LOCK, LOOK FOR THE HIGHEST PIPE AND SEE IF THERE IS A BLEED VALVE, IF NOT TAKE BOTH PIPES OFF THE MATRIX FEEDS AND FLUSH OUT TO SEE IF IT IS BLOCKED, AND RECONNECT LOWEST PIPE FIRST AND FILL SYSTYM TILL WATER APPEAR FROM TOP AND RECONNECT RUN MOTOR, AND KEEP TOPIN UP.

CHECK THE HIGHT OF THE HEATER MATRIX IN THE CAB , IF ITS HIGHER THAN THE FILLER/EXPANSION TANK. THEN ITS AN AIR LOCK, LOOK FOR THE HIGHEST PIPE AND SEE IF THERE IS A BLEED VALVE, IF NOT TAKE BOTH PIPES OFF THE MATRIX FEEDS AND FLUSH OUT TO SEE IF IT IS BLOCKED, AND RECONNECT LOWEST PIPE FIRST AND FILL SYSTYM TILL WATER APPEAR FROM TOP AND RECONNECT RUN MOTOR, AND KEEP TOPIN UP use a flushing solution

If you raise the front of the vehicle (ramps or 4x4's) so your fill is the highest point this should help purging the air out. Also make sure you have heat on high and let the cooling fan cycle on and off a few times while topping off so you know the thermostat has cycled (opened & closed). don't forget to fill the overflow to the right height.