I tried to search the archives before posting and see similar issues...but not the answers. I expect folks don't necessarily make a final post once their problem is resolved. At any rate...I have a 2005 4Runner with the 4.7L engine and 88,500 miles. My heater started giving me problems last winter so when I had the 90K service done this week, I had Toyota check the heater issue too. Since I had asked them to replace the timing belt and water pump, they replaced the T'stat too. It still doesn't work right. Specifically....it heats ok when you are moving but blows cold when you come to a stop...so it is related to RPMs somehow. Coolant is fine, properly bled I assume, new water pump, new T'stat. The discharge temp in the outside vents is 120 degrees but Toyota says it is supposed to be 140. The tech called Toyota and they said to replace the heater core for $1500. The tech says he doesn't think that will fix it. Any ideas? Seems that even when I get 120 degree air out of the side vents, the center vents and rear vents tend to be cool as well. Thanks for any advise you can give. I can check easy things but I can't flush heater core and stuff like that :-) Lotta

 

Just came home for lunch and played with it a bit more....of course it's 68 degrees outside now...it was 45 this morning. In park in the driveway with engine running, the outside vents are putting out 120 degree heat. When I switch the dial to floor, I get heat there, floor and vent...I get heat at both, defrost...it goes cold?!? Also, there is now a whine the comes on with the fan that wasn't there before they worked on it. I have to have defrost! Anyone have any clues? Doesn't sound like heater core to me.

 

Double check your coolant. Do you have any in the overflow tank? I had a car that did the same thing (cold air at idle) and the coolant was low. I filled up the overflow tank and problem solved.

 

I had a similar issue on a Ford F150. It would do fine until I came to a stop and the engine idled.

On the F150, there was a small check valve in the vacuum line going to the HVAC controls, which has a vacuum line going to the vacuum operated coolant valve that opens to let coolant flow to the heater. The line went to the mode selector switch (heat/defrost/vent, etc.), and vacuum was used to control different functions, such as switching to defrost.

As engine RPM drops, the amount of vacuum drops. This check valve keeps the vacuum feeding the HVAC selector from leaking back into the engine and keeps the HVAC controls from changing state as engine vacuum drops.

The check valve on my F150 went bad, causing symptoms like you describe.

I don't know if Toyota uses something like what Ford uses, but I mentioned it because the symptoms sound very similar.