Heater blows cold at idle but warm when higher RPM's..??

Mine was a clogged heater core. Did a very thorough coolant flush and ran CLR through the heater core. Now my heat is working very very well

 

I had issues with my heater that started a few months back (before the winter started - no heat all winter!). The fan would blow, but no heat came out. I went all winter thinking it was a mix door issue, even took my entire dash apart to see if I could get to it. I gave up after a couple hours and put it back together. Then I noticed a couple months back the engine temp would come up to overheat randomly. It got progressively worse, lately, and A/C would stop working when it got too hot. It also failed inspection with a P0117 code - Electronic Coolant Temperature sensor below voltage (overheated). I changed the thermostat on the advice of a mechanic, but it didn't resolve the issue. Last night I stopped at Wally World and bought the Prestone Flush 'n Fill Kit and the Prestone Radiator Flush and cleaner. I installed the flush kit (it apparently had one before as the heater core line had been spliced) and followed backflush procedure on the packaging after draining coolant. Then I followed the directions for the radiator flush and cleaner and immediately saw results. The heater blew hot and the coolant temp stabilized. I left the cleaner in for a few days, as per the "heavy cleaning - neglected maintenance" instructions, and will flush again and replace water with coolant in a day or two. Very happy with the results, so far.

 

Fwiw my truck is also doing this. I don't use the heater much in South Louisiana, but I did notice that it was blowing cold at a stoplight. It also overheats if I sit at idle for too long. If I rev the engine the temp needle comes back down along with the sloshing noise behind the dash that others have described.
I should probably run the same flush / neglected maintenance thing the last poster suggested.

 

Not sure if this was suggested because I didn't read all of the posts...

Check the water pump. Not sure if Toyota uses a water pump with metal or plastic impellers but I have seen the plastic fins break off other makes and this occur. At low rpm the pump will spin but there is not enough fin to push the coolant through. Once you hit higher rpm's there is just enough fin to push the coolant through but with lesser force. I would guess that the heat is not as warm as it used to be even at higher rpm's.

 

I'm going to go ahead and flush my cooling system soon since it's never been done.

The rpm difference though makes me lean toward water pump or fan clutch. I've just been watching it and turning my a/c off if I have to sit in traffic for a long time.

 

I got the crusties

 

With your truck running take your radiator cap off and watch for bubbles. It may take a couple of minutes so don't rush it. That is exhaust gasses escaping.