'92 940 turbo wagon. Recently overhauled the cooling system. New water pump, radiator, t-stat, hoses. Replaced the heater control valve first. Everything is connected correctly. The heat won't shut off. The control valve is working properly, lever in correct position so vacuum is not the issue. The entire A/C system has been removed including condenser. The heat continues even with one of the heater core hoses manually clamped off. Blower motor is new and not contributing to the problem.

If the car is started and allowed to get up to full temp w/o moving it blows at ambient temp. Drive it and the heat increases incrementally until full temp.

The turbo creates enough heat to make the passenger side firewall pretty warm. Could this heat move by conduction into the heater core? Since this problem existed prior to the above maintenance and was the reason for changing the heater valve that seems implausible.

This is no big deal in the winter but in the warmer months it makes the car almost undriveable.

Theories?