Any Toyota Techs on here?

Warm it up with rad cap off. As thermostat opens it should be a pretty good flow past the opening. Or swap the pump.
I have tried burping it. My driveway is a hill. Parked in the middle, took the cap off the radiator. Let it run for like 20 25 minutes.
You definitely cannot count on that to purge all of the air trapped in some systems. Take the
highest coolant hose in the system off and see what comes out, and with the cap off if the
level in the rad starts dropping when You do.
As far as the heater control valve, can't remember on that year, but most are vacuum control.
And most are designed to default to open so that You have heat to safely defrost the windshield in the winter, so unless the controls are holding it closed by some malfunction, it
should be wide open. How do the hoses feel temp-wise in vs out?
As TJ said, even at an idle, there should be a reasonably brisk flow of coolant visible in the
rad w/the thermostat open.
The best way to bleed a system.

I mentioned the vacuum idea because of a few reasons. You asked for a Toyota tech. I worked for Toyota in Macon, Georgia for a good while. I am still Toyots certified, as their certifications never expire.

Secondly, I left there in 1994 and at that time, there were some HVAC systems using vacuum.

Third, vacuum hoses, if the HVAC is equipped with them are dirt cheap to replace. I always try to recommend the cheapest thing first, because 1) the cheap stuff tends to break or wear out first and 2) it's cheap to buy. lol

Not all systems will default to the defrost if they lose vacuum......even if they are designed to. I have found that out the hard way. On more than one occasion.

At any rate, after reading everything you've tried, I agree with making sure the pump is flowing good and there is no air trapped in the system. Either condition could certainly cause what you have. Posts 26 and 27 are right on the mark, I believe.

One last thing to consider. If the fan clutch is faulty, it can be faulty both ways. It can freewheel, but it can also lock down as temperature increases. If that's the case, it can actually cool "too good" and cause the heater not to get hot. That's a long shot, but certainly worth checking. The Toyota fan clutches are serviceable. They can be disassembled and refilled with the silicone gel that makes them work. I have seen some so dry, they lock down, rather than act as a clutch. Just something to check.

Good luck!
The heater control valve is cable operated. The hoses are hot.
You might want to verify the air doors are actually connected to their servos. Highway speed/air pressure thing going to push air past or close doors.
Got a update. It was heating good enough to get by. However yesterday while driving on the highway it started blowing cool. WTF?! I got to my destination did my thing. Got back in and its blowing warmer now. Still not as good as it was. I thought I had replaced the heater control valve but, i didnt. I did throw a used one on there to see if it made a difference, it didnt. Gonna put a new one on today. I'm not optimistic. After that I got nothing.
Cabin air filter. That would be the first thing.
Makes lots of heat but wont clear windows. Easy problem to miss.
No cabin filter. 2003 and up.
Hot hoses on heater core, air blend door on hot and fan working should make heat.
Add a leaking head gasket into the mix,enough to airlock the heater core and theres your problem. A highway drive usually brings this problem to the surface.
No cabin filter. 2003 and up.
Hot hoses on heater core, air blend door on hot and fan working should make heat.
Add a leaking head gasket into the mix,enough to airlock the heater core and theres your problem. A highway drive usually brings this problem to the surface.
I don' see any leaks nor am I losing coolant but, that makes sense.
How do i check the head gasket? Have the system pressure tested?
All it takes is a tiny leak,compression into cooling system creates an airlock. Low rpms and its normal, highway drive high rpms increases amount of (air) in coolant. I have a test kit that detecs combustion gases in coolant for this very reason.
I use this.
Its a chemical test,will detect even the smallest leak. It samples the air in the resevoir, and chemical changes color.
Fan clutch has nothing to do with it. Thermostat controls temp. What is gauge reading when heater blows cold?
Fan clutch has nothing to do with it. Thermostat controls temp. What is gauge reading when heater blows cold?

My thoughts too. Fan is certainly a part of it all, and a proper clutch can gain you some fuel mileage and better acceleration. I like direct drives on big blocks. Most of what has happened to this car has actually improved the cooling system. And that is beneficial. I see no reason to try and defeat the system. Just do what I have done for 45 years. It's free. A piece of cardboard. When temps get close to 40 F toss it.
Up here i have ran 205 degree thermostat and rad covered with fan removed for a few weeks,im pretty picky when it comes to a warm vehicle.
Gonna check the fan clutch today.
Stop, just stop. Before any other checking things, lets establish some current conditions;
1) What is the engine temp throughout these "ups & downs" in heater output?
2) Do You or don't You observe coolant flowing freely through the rad w/the cap off & the thermostat open?
3) You stated matter-of-factly 3X You'd changed the heater control valve, then said oh, no I didn't. You also said it is mechanically(cable) actuated.
4) When You did change the heater control valve w/a used one, was there any obvious defect or signs it had failed? What about the used one?
If there isn't any broken/loose/frozen/missing component of the valve.....what would be wrong with it? Is the cable working freely and as intended?
I've had ends corrode and prevent the cable from moving thru it's full range, some less than half, because it's unable to pivot. Do the controls move freely?
5) A locked clutch fan is simply a solid fan, the thermostat isn't going to let any "overcooled" rad coolant in if the engines not up to temp, the bypass will still
let the heater have coolant flow aplenty.
6) Have You pulled a high-up coolant hose off yet? If there are small coolant hoses at the throttle body to prevent icing, the top one is a good choice if poss.
7) If You've pulled a hose, and are sure that air is not trapped in the system, then have an assistant rev the engine and heat it up. Watch the coolant reservoir
for bubbles, then have them put it in gear and 1/2 power block it against the brakes for 5 sec. or so, let off, neutral and rev again..watching for bubbles. A
bad head gasket is going to put a steady pattern of gas bubbles into there, and a nice rush of them under load.
8) Why did You change the heater core to begin with, and what was the apparent condition of it & the coolant/system when it came apart? Was it plugged?
9) I've had people that threw things on the dash, pens etc. and they went down the defrost ducts, jamming the blend doors....not always the same as vehicle
movement/attitude changed & the heat selector moved the door back & forth. Cost a Guy w/an F250 a pretty$$$ 'cause His farmhands did this, I retrieved
a Bic and a plastic knife, LOL.
10) Is the engine temp performance consistant? I just installed an aftermarket T-stat('cause our crack dealer parts no-gotsa), and it didn't come with a bleeder
"dingle" valve. That means there is a tiny v-notch in the valve disc, so I looked for it to be sure it was up top,......the wax pot was skewed and it wasn't even
shut!!!! I had to put My pocket screwdriver in and center it in the block-side frame, it looked OK and heated fine....and no return of the T-stat perf. code so........
11) If You have decent heat & suddenly lose it while driving, it can only be these;
A) A pocket of trapped air or leaking combustion has entered the core
B) Something has obstructed flow to or from the core
C) The blend door or control system has suddenly moved by itself, or the door isn't seating/sealing tightly
D) The engine temp has gone below operating temp for some reason
E) Something is obstructing airflow thru the core
Not seeing this system, and having not worked on that HVAC unit in over a decade probably, .......................................................................
I've seen some rides that had so much crud in the block, they perpetually clogged up heater cores, back-flush 'em and go's like a freakin' filter!!
Killer pretty much has it down, changing a heater core in a polluted system will result in another plugged core.
Could it be as simple as a faulty thermostat? First it was hot on the highway and cold at idle, now it's hot at idle and cold on the highway. Changing symptoms means something is behaving erratically.
The engine temp is where it should be. I put a NEW control valve on it. I did the heater core because I thought it was the problem. I have also done the water pump and timing belt. I finally broke down and took it to a friend who is a retired Toyota tech. He is also stumped. By accident he noticed a small threaded rod that goes into a plastic arm. He adjusted it a little and heat got worse. He went the other way and it got better. He has no idea what it does. The heat has been pretty good until yesterday. Im gonna scew with that rod today. It on the passenger side under the heater box. There were no signs of corrosion in the other control valves.