1994 Plymouth Voyager

The heater is a technicians way to test the overall condition of any vans cooling system while roadtesting a van for any cooling related problem. With 138,000 miles it could have a leak and losin coolant, as the level goes down you lose your heat first, and the steam and overheating comes next. Fill it up, pressure test it to find out where the leak is, the head gaskets will slowly burn up the coolant, you won't see an obvious leak but at high mileage its a problem that is common. You may have a thermostat problem also. When I do head gaskets a new thermostat is always part of the job.