Any Toyota Techs on here?
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 again.....it's like a freakin' filter!!