Hello. 1994 Volvo 940 non turbo. A few days ago temp needle started climbing. I stopped. Checked coolant level. Was below minimum. So added coolant. ~ 25 miles later same thing. I have looked really carefully at all hoses & fittings and can see no dripping or leak. Done with car running.

I only checked engine compartment area/under hood. Could leak be from heater within the car ? Thinking maybe as coolant passes thru there. No wetness on either driver or passenger side.

Not sure but I think what may be heater valve which is on one of the two coolant hoses that pass thru firewall into car is not working. Valve does not move when turning from cold to hot air. Shouldn't it ? Could this need replacement ?

Thanks in advance for any info & advice with this problem.