heater help

[steeltowninwv]

just got my 4runner and thought i would check and see if the heater was working, i drove it to the store and kept it running while i went in...cranked the heat up...when i came back out it was blowing cold air....soon as i went down the road it was blowing nice hot air?................also my heater makes a loud humming noise when running...well im sure its the fan cause ac makes the same sound?..any way to quiet it down....forgot to mention it is a 95 4runner...ok i just went to the store about 13 miles away....after the vehicle warmed up i kicked the heat on....warm hot air like it was supposed to came out.....thenabout 5 minute later i stopped at a red light..it turned to cool air while i was at idle...when i took off again almost instantly it was hot air coming out again....when i got home i let it idle for like 1 to 2 minutes in the driveway....air turned cool again...so i have nice hot heat except while i am at idle.....any suggestions?

 

[fourwd1]

Sounds like coolant isn't circulating thru the heater core properly.
Maybe you're low on coolant, maybe the water pump is on it's way out.

 

[the_dingman]

Remember that the thermostat in the engine restricts the flow of coolant when the engine is not at full temp. So if the thermostat had not yet opened, it would not blow hot air. That may be the first thing to replace, as it's about as easy as it gets to replace, and costs very little.

Another thing to know is that there is a bypass valve on the heating system that is supposed to close when you slide the heat control to 'cold'. If this is sticking, as it commonly does, it could be restricting flow at low RPMs.

There is also the possibility that there is just a lot of crap in the system, and there was a short plug. Remember that you're working with a very old truck. Try flushing the cooling system.

You could also have a horrible, catastrophic problem with your engine... Are you having any overheating issues? Where does your temp gauge sit in normal use?

 

[fourwd1]

Remember that the thermostat in the engine restricts the flow of coolant when the engine is not at full temp. So if the thermostat had not yet opened, it would not blow hot air.

Umm, not quite.
The t-stat controls coolant flow thru the radiator. When it's closed coolant flows thru the engine and heater cores, allowing everything to come up to operating temp quickly.
Once that happens it opens so that temp can be held constant.