The dash temp gauge reads in the middle between hot and cold within a few minutes of running. However, it takes about 20 minutes for hot air to come out the heater vents when the heater is set to maximum heat and the fan is on. What should I check and in what order? Thanks for any help.
I suspect that the heater core is partially blocked.
- Coolant level in both raditor and via bottle. Low coolant will exhibit this exact symptom.
Had this issue back in college I put up with one cold winter and finally gave in went to mechanic for my $300 rotted Subaru XT coupe. He charged me $10 to fill the coolant 
although bottle read okay.
There was enough coolant for the car to work okay and temp to read. However the low coolant had to heat to start flowing into the heater core well enough.
After you top off the coolant, warm up the car, turn on the heater and check the heater hoses. They should both be too hot to hold on to. Some cars have a valve that cuts off flow to the heater core when using the A/C. I have seen those valves stick shut.
Thanks all - I will check the coolant level first, since I know how to do that
When you were sitting at idle you confirmed that the lines coming thru the firewall to the heater core were hot. And you did get the radiator fans to run, so those two things tell me that fluid is reaching full temperature at least for a while. After that quick spin in which the cabin heat dropped off, did you check to see if the heater core lines were still as hot? If they dropped in temperature substantially, I’d say that your thermostat likely has a problem. If they are still hot and you don’t have heat coming out of the vents, then the problem could be within the HVAC box itself.
There are 3 servo motor controlled flaps:
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Recirculate / fresh air
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Mode Damper - determines where the air flows to (def / dash vents / floor). Sorry - I said Mode Damper in my reply, but I meant Air Mix Damper - see below.
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Air Mix Damper - routes air thru or around the heater core. XLE/limited vans actually have two of these for left and right front control.
I suppose that an electrical position command or a mechanical linkage problem could change the flap position and retard airflow thru the heater core. Exactly why will take some work to determine.
If your heater’s hot-cold control operates a valve in the plumbing to the heater core, make sure that valve is open (heater set to Hot) when you fill or check the coolant level.
If I had this problem and nothing else seemed to work I’d temporarily remove the hose connected to the outflow of the heater core at the fire wall (from the engine compartment side) and see if any coolant was coming out. That way at least you’d know if you had flow through the heater core or not. If no or little flow out the heater core it’s possible the heater core inlet valve is not working, the heater core itself is plugged, or, more likely, there’s an air bubble blocking that pathway.
I filled the coolant using the reservoir up to the “full” line. Tried running the van, took it back up to normal operating temperature. No change (still no heat). I then located the heater core inlet/outlet tubes. The easiest access to them as far as I can tell is from inside the van, they come in to the right of the gas pedal. I touched them, one was very hot, the other was quite hot but I could hold on to it without getting my fingers burnt. Does this give any more clues?
With that information I would say you have two possibilities. One, the heater core is blocked so the normal flow through it is being restricted or two, the heater core is okay but there is a problem with the blend air door that selects air between the AC cooling and the heater. I would tend to side more with a problem with the door than a blocked heater core.
Cougar - thanks for your comment. Somehow whatever the problem is seems to clear up by itself after 20 minutes (the heat comes on at that time).
If the heater core was blocked…it would NEVER blow hot air… The valve that controls the amount of coolant flowing thru the heater core is more likely the problem…
Older vehicles had a cable operated valve that let coolant into the heater core, slowed it down, or stopped the flow completely. I suspect the motor that controls this regulator valve is either slow to move or is unable to move the valve until the coolant warms the valve body and makes it easier to Open the Faucet if you will and let the coolant flow …
Thats where I would start…those stepper motors die all the time… also the valves can sometimes have an obstruction which makes it hard to open and close easily.
Blackbird
Blackbird - thanks for your comment. That might explain why the output line from the heater core is hot, but not too hot to touch. Perhaps there isn’t enough coolant flow to make it really hot until the valve opens (?). I guess my next step will be to try to locate the regulator valve/stepper motor assembly.
'I filled the coolant using the reservoir up to the "full" line'
Does this vehicle have a radiator cap?
If it does, merely filling the overflow reservoir is not sufficient, as the radiator could still be low on coolant even though that reservoir is now full.
When the radiator is stone-cold, remove the cap (if it has one) and fill the radiator to the top of the filler neck. Then, drive it and see if there is a difference in heat output.
I have ordered a Haynes manual, hopefully that will give me some good info as to how to get to that part.
^
If this vehicle has a radiator cap, it will be located right on the top of the radiator.
While a Haynes manual is a good thing to have, you shouldn’t need it in order to locate the radiator cap.
Don’t bother looking for a heater control valve. Your vehicle doesn’t have one.
The coolant flows thru the heater core at all times. What controls the amount of heat that enters the passenger cabin is with the air mix damper or blend door.
This door is controlled thru the HVAC control module. The HVAC control module determines the position that the door should be in from the information it receives from what Toyota calls Room Temperature Sensors.
http://toyotaparts.lagrangetoyota.com/showAssembly.aspx?ukey_product=43398083&ukey_assembly=5736884
There’s three of these sensors inside the vehicle. If one of these sensors is giving a false reading to the HVAC control module, it confuses the module as to when heat should be introduced into the passenger cabin, how quickly, and how hot.
If you look at the factory service manual, each of these sensors has a resistance value related to temperature.
But don’t expect to find this kind of information in a Haynes manual.
Tester
Thanks for the additional comments. I added coolant directly via the radiator (not reservoir) and it took quite a bit more coolant. I then started the engine and revved it at about 3000 rpm for a while until the temp gauge was between C and H. I felt the heater core input and output hoses and they were both very hot (too hot to keep my hand on for more than a brief moment). I turned the blower to “hot” and initially it came out hot. I thought it was fixed. But then the air temperature dropped down to warmish again, so I guess not. There was just an initial blast of hot air. If this helps, I have the “CE” model (the most basic model of Sienna). Based on what you wrote Tester maybe I should be looking at a possible malfunction of the air mix damper or blend door and related sensors. Does my model of van have these sensors? As far as I knew, I just crank the dial to “hot” and it comes out hot (at least, it used to). If needed, how do I get hold of a factory service manual so I can locate and check out the sensors? Thanks for all the help.
If your cooling system was very low you will need to top off the radiator again after operation the engine. You may have a water pump or radiator leaking, keep an eye on the cooling system.
It is a good thing you have a need for the heater, if you lived in the desert the lack of coolant in the engine may have led to complete engine failure.
Sounds like you have a cooling system leak. That’s a bigger problem than the heater not working.