Hugo

Lately, I've noticed that my heater seems to have some sort of problem, which occurs especially when the truck is heating up, but also when it is warm.

Say I arrive at a red light and the heater is on (it's cold here this time of the year, after all). Well, the minute I stop rolling, the temperature of the air coming out of the vents starts dropping fast. By the time the light turns green again, it's blowing ice cold.

Same thing in stop and go traffic. The air starts blowing cold in no time. Yet as soon as I accelerate or otherwise put a load on the engine, it blows warm again. Putting it on the recirc position doesn't change the situation, it seems.

What can be causing that?

eric-the-red

Low coolant flow at idle will cause that. First check the coolant level, make sure it's not low. If that's OK it could be your heater core is getting plugged, or your water pump is going.

marko3xl3

Your thermostat might be bad, because if it keeps coolant circulating when it's not needed, you'll cool it too cool = no heat.

MTL_4runner

First thing is definately to change your thermostat, do a coolant flush and refill to the proper level.

midiwall

yeup yeup yeup. Add a vote for the thermostat to be stuck open.

MTL_4runner

You might want to read my thread too, sounds very similar.
https://www.yotatech.com/forums/f2/those-cold-climates-hows-your-heat-74448/

Make sure to refill with OEM Toyota coolant.

Hugo

Thanks guys. I will get a new thermostat and do that. I need to change my oil, and we're babysitting a friend's baby this weekend, so getting out of the house is an *excellent* idea.