On the Astro and Safari vans, there is a vacuum control valve on the engine side of the firewall that controls the flow of water to the rear heater. Looking from the front of the van, it is on the left (passenger) side of the engine about 12 inches below the hood line. When you turn the rear heater on, the valve opens to allow hot water to go to the rear heater. If your van has lost vacuum pressure, the valve will not open properly. You will probably also notice that the airflow director for the front heating/cooling system will not change positions. This loss of vacuum pressure is usually caused by a broken or disconnected vacuum line. Find and repair the break or reconnect the line and this should restore function to both systems. If the airflow director for the front system is working, then the valve itself may have failed. You can either replace the valve. A quick fix is to remove the water lines from both sides of the valve and use a 3/8" plastic connector from the hardware store to connect the two lines. Use two clamps to secure the hoses to the fitting and you should be back in business. For a picture of the valve, go to http://www.napaonline.com and search for part # BK 6601320.
VENT TO HEATER CORE IS NOT CLOSING................
An automobile's heater that blows only cold air could either be a faulty thermostat or, more likely, a faulty heater core. A trip to a mechanic or shop that specializes in radiator and cooling system repair should be able to diagnose and repair any problems.
the water control vale could be broken or have a broken vacuum line to the valve. If it is cable operated valve the cable could be broken or jamed. the heater core could be pluged
Low coolant? Wrong thermostat temperature range? Heater core plugged,restricted or airbound? Water pump not circulating coolant? Temperature control cable not adjusted properly?
You are likely in an antarctic region.
You probably have a bad thermostat.
You may be low on coolant, or the thermostat is stuck open, or the heater core is plugged up, or the temp blend door is broken.
It could be low coolant or it could be that the water pump impellers are rusted or worn through.
Could be nothing more than a bad fuse Could be a bad blower motor Could be a bad switch
The most common issue is the heater / climate control valve. This can usually be tested in dual mode and checking for change in air circulation.
There is something wrong with the compressor or there is no refrigerant in the system. Have it checked out. Goodluck Joe
the heater core could be stopped up, or could be low of antifreeze, or the thermostat could be stuck open
I assume your car heater is blowing cold air, when the heater is on. It seems that, you have air on your coolant. that air pockets blocks the cooland to travel through your heater. it may be a sign of a bigger problem such as blown head gasket in the old cars, such acura legend
could be thermostat or heater fan if you can hear all the speeds on the blower motor working then it could be your thermostat but your heater core might be plugged as well
Check the heating elements in the water heater. These may be bad and need replaced.
Mercury Mountaineers have a "blend" door in the heater box. It is probably malfunctioning. You can find more info by searching for Ford + "blend door" or "no Heat"
check you thermostat for the temp setting
AnswerReplace the blower resistor.
See "Related Questions" below for a complete discussion on possible causes and more importantly: fixes
I'm not a mechanic / technician but it sounds like your engine cooling thermostat is sticking open ( assuming that the electric engine cooling fans are not running continuously )
Could be a plugged heater core. Check the engine temp guage. if it is getting warm, your heater core could be plugged or restricted.
I would expect a plugged heater core.Take your hand and feel the two heater hoses that pass thru the firewall on the passenger side; if one is hot and the other is much cooler, than you heater core is plugged and needs replacement.
sounds like you need a new head gasket
If you are not getting heat out of the heater, there are only a few things that could be wrong. The first is low coolant. You can't get heat without coolant running through the heater. If it is low, sometimes it has to be bled of air bubbles. If both heater hose running to the heater are hot, you have circulation. If not the heater could be plugged or not getting the coolant to circulate. There is also a damper in the heater system to stop cold air from bypassing the heater and that needs to be checked.
If a hot tub is not holding temperature there could be several things wrong. The heater function has stopped working or the thermostat is faulty. Also check to make sure the heater has sufficient fuel. The thermo coupler on the water heater may be faulty.