In that case, you should indeed check the monovalve. It is a square looking valve near the battery on the firewall. If you remove the four screws and both sides of the diaphragm are "flooded" then you need to get a repair kit and replace the diaphragm/plunger unit.
Also, remove the glove box and look behind it to see that there is a foam hose going horizontally connecting the temp sensor with the horizontal pipe. If it is gone, get some foam pipe insulation at Home Depot and craft a new hose.