No gurgling sound and haven't done a radiator service but will do. Thanks.

Thanks guys. I'll give you an update

So I am having the same issue, and I've run through a few checks. Originally had the running water sound in the engine, and I've been keeping the fluid levels up, then had a working heater for like a week. Now it's back to the lukewarm from the front ones. The thermostat seems to work as it shows the correct engine temp, the tubes running to the heater valve are nice and hot, the valve opens and closes, and the back floor heater works quite well. That's as far as I've gotten, query as what to do next, seeing if people have had solutions.

maybe the blend door. lay under the dash and cycle the hot/cold controls. watch what moves, and doesn't move. it sounds like a control cable has popped off. I cant remember which side the blend door is on, there are controls on each side, so check them both.

I'm banking on a clogged core... for some reason I see so many issues with them in these trucks.

I made a super simple tool for about $5 at Home Depot and it works awesome. 10' roll of 5/8" clear vinyl tubing and one brass garden hose thread x hose barb fitting. Cut the hose into a 2' long pressure line and use the remaining 8' as a discharge so you don't get hosed. Hook up to heater core and let run full blast for a few minutes, then reverse the hoses and do it again.

You'd be surprised how much trash usually comes out...

Thank you guys for the help,
I did take a look under the dash and everything seems to be moving alright. The two oddities I want to mention first are I did have heat for like two weeks or a week before it went back to lukewarm, also, the rear back seat floor heaters work well. But I am thinking it is the core cloged with gunk after looking through the thred. How do I get to said heater core though?

You can access both heater core connections for flushing right in the engine bay at the firewall. As far as changing said heater core, well that's another story...

Hey guys, new to the forum, I'm having nearly this exact issue with my 2003 4runner SR5.

Just bought it and only noticed the issue this morning.

Air is warm when accelerating but cools down very quickly as the engine winds down. Almost cold coming out when I'm sitting at a light.

I've read so many posts that say so many things(heating core flush, water pump, low coolant, aftermarket stereo), I just don't know where to start.

I've got coolant full in the reservoir, first thing I checked.

In one post I read something about the radiator hoses being different temps.

My in and out tubes in the radiator are not the same temp. One hot one not.

I am not a car genius, any help would be appreciated.

You can get a bottle of coolant flush at your local parts store or online, very easy to use, just follow directions on it. It's important to use distilled water to prevent deposits, and the right Toyota coolant to prevent corrosion. At dealer or online, aftermarket brands make some that are supposedly compatible but boy worth the few bucks saved imo. If someone removed the thermostat that would also keep car running cooler but shouldn't affect it as quickly as to be noticeable at a stop light so clogged core would be my first guess also. And easiest to fix

Just had the heater core flushed today and the guy said stuff came out and he declared it fixed.

After driving for a few minutes to warm it up it was pretty clear the problem hasn't been fixed. It's still inconsistent.

Sometimes warm, sometimes not. High fan speed and foot well seems to not work at all. More warm sure while foot is on the gas.

Is this thermostat, radiator, water pump, or something else?

just had this issue, first we found the thermostat was stuck open. got better but still not hot. then found core was plugged. you could also have a heater box door issue