Heater not working?

2002 3.0 V6 Auto. Tran. When I start my truck to warm up, no warm air is blowing until I am on the road. It doesn't matter if it idles for 5 minutes or 30 minutes. Yesterday while driving, I lost heat, cold air started to blow, and my engine temp went up as if it was starting to overheat. What could the problem be?

It turned out to be the heater return line going from the water pump to the expansion tank and the heater core. The outlet for the heater core line was completely plugged solid.

Had to take off the A/C compressor, the alternator, the bracket that holds the tensioner. The exhaust manifold, the air intake. Basically the entire engine.

The hose was pretty much a nightmare to repair but it solved the problem.

Up until a couple of weeks ago I had heat, no longer. I thought it maybe the thermostat but its not. My top radiator hose gets hot, lower hose warm, my inlet hose to heater core hot, but the outlet hose doesn't even get luke warm. Could my heater core be clogged?

Is there a way to tell if my water pump or termostat is not working? My temp gauge does not register any temp and I have no heat. From various sources on the web it seems like it may be either the thermostat or the water pump. Before I replace one or the other I would like to narrow down the problem. Also if it is the thermostat I have conflicting repair solutions. Some say I can just replace the thermostat other say Ford made a change and now it is not possible to change the termostat beacuse it is part of a larger more expensive component. I didn't have a repair manual but have bought one and it is in the mail.

Thanks for any help.

I just recently noticed the heater isnt working like it should. The blower is working fine and its pumping the air out fast but its not really hot air. More like luke warm. Before I could start the truck up in the middle of winter, let it run for 5 min and when you got inside it was really warm. Now it takes about 15-20 mins just to get it a little warm. Even when driving it after letting it warm up, it never really gets warm to the point that you have to turn the heater down. Its slowly gettin worse and worse. At 90,000 miles and never having a problem, im not surprised something is starting to go bad so I cant complain about the truck. So is there something I should look at first to solve the problem? My guess would be the thermostat?

Well, the temperature gauge doesnt work, hasnt since last winter. I had called a local auto repair shop in town to ask them if it would hurt anything leaving it the way it was, and they said no as long as I am checking the coolant routinely. I only drive the truck in the winter time, and its stored in the garage pretty much all summer. When I told him that he said dont even worry about the gauge seeing is how I barely drive it. But I do know that last winter the heater worked really well when the temperature gauge wasnt working. Now this winter something obviously went bad because its not putting out that much heat.

My truck starts out cold even though it is garaged over night. The gauge is at the very bottom of the dial and sits there for at least ten miles of driving. The gauge finally starts to climb and then pegs out and the "check engine" light comes on for a second and then the gauge drops back down to normal. The heater blows cold air out even though the gauge says the truck is warm. The heat starts to blow out of the vents after about three or four miles of driving. When stopped the overflow reservoir is topped out and the cap is usually popped off. The radiator is about one to two quarts low then. I have had the system flushed and a new thermostat installed. The hoses are warm to hot when the engine is running. Could it be the heater core not operating correctly?

I took it back into the shop where I had the system flushed and thermostat replaced. They said it could possibly be the head gasket. I will let you know their findings.