1993 Plymouth Voyager • 150,000 miles

I have a 1993 Plymouth Grand Voyager SE. I'm having trouble with the heat. I drive approx 30 minutes to work every morning and I don't get any heat. I've watched the temp gauge and it barely moves during the entire trip, staying pretty much all the way left of the gauge. I've noticed if I sit in a parking lot with the engine running for 15-20 minutes, the guage starts to move and I can get heat. But if I start driving, after approx 5 min. The guage starts to drop back to the "cold" side and the heat goes away. I've never had a problem with the van overheating, and I checked the antifreeze level and it's full. I have VERY limited automobile knowledge, but would appreciate any help I could get.
January 5, 2013.

After driving a good ten miles or more, feel the upper radiator hose. It should be too hot to hold onto for very long. If it is not, suspect the thermostat. Follow that hose to the housing on the engine. It will be held on with two bolts.

Jan 5, 2013.