2007 Sienna heater blows cold in front and warm in back??

It could be any number of things, including a bad thermostat.

Step one is to determine whether the cooling system is settling in at the proper temperature. Is it getting hot enough to make the radiator fans run? Do you have an OBD tool that will allow you to read out an accurate cooling system temperature? If it isn't reaching a little over 200' F when fully warm and under some load, then the fault might be with the new thermostat. If it is getting to temp., then there is always the question of air in the system and a pocket high in the dash that's causing a lock on flow to the heaters. Next is to check that heated water is reaching the heater cores. Up front you can feel the two lines just above and to the right of the gas pedal coming thru the firewall and going into the HVAC box. Silt from using the wrong coolant or hard water can indeed block the core. If hot water is circulating thru the HVAC box, then you have to see if the mode damper is allowing air to circulate thru the core.

The two lines coming through the firewell are very hot. Too hot to touch for longer than 1 sec.

Fluid gets hot enough the trigger the radiator fan and the hoses are hot but I do not have a way to measure the actual temperature of the coolant.

I idled the van at 2500rpm for 5 minutes while the front side with the jack on each side to the try to encourage any trapped air to run out the overflow. The rear heater works just fine. The front heater got warm to hot but would not stay hot when I drove down the road and back for a quick spin.

My wife ran the van to kids school and the store today. When I got home and after the van had cooled off the level in the reservoir was about an inch lower than when I originally refilled the coolant.

The mode damper... what does it do?

The van has 123,000 miles on it if that helps.

I am thinking I have a clogged line somewhere but I am not a very good DIY guy though.

Thoughts?

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:

1) Recirculate / fresh air

2) 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.

3) 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.

Well I broke down the took the van to my trusted mechanic. I did figure out the rear system is separate from the front with a separate control head.

He confirmed your suposition that it could the "blend door motor" as he calls it and apparently there are 2 in the front or the control head. He said a door is not moving when asked to that directs the air to either the evaporator or heater core. He can not rule out the control head failure too because he doesn't have the scanner to scan that part of the system as that type of scanner is propeitary by Toyota. He has ruled out the thermostat since hot water is getting to the rear heater core. He has given my van back, without charge, since he can not fix it so I can send it to a Toyota dealer to have it properly diagnosed and pinpoint which damper as you call it or the control head to fix.

I would like to fix it myself but without the scanner I don't know if it will be possible to isolate the problem. I also know removing the dash will be a pain.

I need to know if anyone knows of any instructions or advise for removing my dash and if the problem can be isolated with a volt meter instead. First times are always slow. Any advise is greatly apreciated as I am trying to avoid a $600-$1000 labor bill as I have confirmed.

The temperature controlling door is referred to as the air blend door or air mix damper door, depending on who you talk to.

So do you have an automatic system (left and right temp controls - 2 doors and 2 motors) or the manual system (single door and motor)? First thing I would do would be to have someone observe the door linkage operation while you drive and see if it is actually changing. If it isn't changing, then why does the temp change? If it does change, why?

If it is getting an unwanted signal to change, maybe you could disconnect the line to the servomotor and lock it into full heat for the winter, and just use the fan to regulate temperature. You probably don't want to start a project like this in the winter! The HVAC box runs about $1700, and the front panel with the AC Amp behind it is something like $1000. Not for the faint of heart, and especially if you are still guessing at the cause.

I posted some system drawings for someone else only a few weeks back, so let me find that link and post it here.

The Toyota dealer has diagnosed and determined the blend door actuator needs repair and will replace it with Servo Sub-Assembly Damper... http://www.toyotapart.com/87106-08060.html . I decided to let them fix it after negotiating the price down $50 to $500 total and knowing they were going to charge me at least $80 to diagnose my problem which is now inclusive of the $500 total. The price reduction came off the part now about $143 so labor fee now will be near $350. Parts and shipping would have cost me $130 if I ordered it myself so thats a good deal.

I was geared up to do it myself but it is not a bad deal as the labor is much less than expecting and I was going to have to take a vacation day to ensure the project remained a weekend project. The wife and kids need the heat working sooner than later. What initially helped is that I have now confirmed that Toyota dealer repair costs can be negotiable. The service advisor I spoke to knew by my description of the problem that I was a DIY person and that likely a large labor fee would not have sold the job. I decided going in that if they could complete the repair for $500 or less than I would let them do it. All I asked was if the price was negotiable after he gave me the quote and he came down to $500 without me asking and further. He also told me they were certain they had properly diagnosed the problem as long as unforeseen problems are not encountered such as something I might have done in the area of the Servo that would complicate their work which I have not.