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Discussion Starter #1 (Edited)
Our 2012 Sienna suddenly has no heat from the front or rear heater, there also was a small coolant puddle under the car. I was out of town with work when this happened. The weather here in central Texas has been intermittently 80's to the 40's. However these problems happens when during "very cold, freezing" nights in the high 20's. My wife said that she first noticed this while she was on a short drive around town picking up kids from school: the temp gauge started to rise without going all the way up, and the "COOLANT OVERHEATING light flickered on then off. I believe it was working 100% correctly last week. She described that the temp gauge would go up under acceleration from a stop; at the stop the temp was normal, but when accelerating from the stop the "temperature budged up, just a little, but still noticeably. And then when she stopped accelerating and reached the speed she wanted to go the temp would stop going up." I think this happened after the initial coolant leak on the cold morning -- so with a lower than usual coolant level.
I was a professional mechanic some 20 odd years ago and closely maintain our vehicles myself and have not observed any leaks in the past, though I rarely drive this vehicle.

I came home, topped off the coolant with about 1 gallon of water, plus filled the coolant reservoir. (I'll add the correct coolant ratios once the problem is sorted out.) I have let the engine fully warm up, and there is no visible leak. I have car up on jack stands and have been closely observing from above and below, and there really is no visible leak. The system seems to be fully pressurized and operating at the correct temperature. It has now been idling for almost 2 hours, the temp gauge has indicated its normal temp the whole time -- right in the middle. During this long idle sometimes the AC was on high, sometimes the heater on full, sometimes with the HVAC turned off, and there still is no leak, the heater is not getting hot, and the temp gauge is not changing.

One anomaly is that there was a dead sparrow lodged into the lower radiator hose's spring clamp, where the hose connects to the radiator. I can't imagine how this would affect things, but it was right at the clamp and really stuck there?! I can't help but wonder if this some how accounts for the coolant leak.

The radiator and radiator hoses are all "hot to the touch," once warmed up the temp gauge has not changed from its normal "in the middle" location.
I have fiddle-faddled with all the front and rear heater controls, hot and cold, AC on and off, fresh air and recirculating... and it seems that front and rear heaters are maybe getting a little warm, but really they just feel like exterior air temp. The cold AC is getting nice and cold.
I visually verified the driver's HVAC actuator is moving correctly -- the one at the driver's feet behind the kick panel. I have only looked at that one (not the passenger side). The steel heater hoses going into that heater core, one is hot and the other is cool or only warm.
Under the van, the heater lines going to the rear heater, one is very warm (not as hot as the main radiator hoses), and the other is cool to the touch.the rear heater is not getting hot, but it is more warm than the front heater.
In the engine bay, where the heater lines come off the driver's side of the motor, both heater hoses are only very warm to the touch -- neither are hot; the upper radiator hose right next to these heater hoses is very hot. they probably are slowly getting more hot as the car idles for hours.
Possible head gasket? -- well, there is no oil in the coolant, and no emulsification on the dipstick, not is there any hint of coolant smell in the exhaust, and I can't smell exhaust nor is there an "exhaust gas volcano" at the radiator cap so I think a head gasket leak is unlikely.

I really am not as familiar with the intricacies of these motors, but it seems highly unlikely both the front and rear heater cores would fail at the same time, suggesting that there might be an issue with the thermostat? Does the heater coolant circuit go through the thermostat on these cars? If not then it seems likely it is the heater core. But then how do I explain the coolant on the ground? Why did the coolant leak out when parked overnight on a 28 degree night, but ow there is not coolant leak when hot and fully pressurized? I know that the coolant had a 50/50 mix ratio, so it is very very unlikely anything froze...
 

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Discussion Starter #2 (Edited)
update.
after another hour of idling, and internet reading, I came across this thread:Heater Control Valve access and testing

where the owner of a 2007 Gen II Sienna (mine is a 2012 Gen III) had success after months of frustration by simply jacking up the front end of their van 6+ inches, removing the radiator cap, and letting it heat up.... this was suggested to him by a knowledgeable service tech/servicewriter, who said sometimes an air bubble can get caught in the system, and changing the angle can help it break free.

So I tried this too, and my heater is suddenly super hot!! Both the front and heaters are not hot, and all the steel heater hoses are very hot.


I still am a bit mystified by the disappearing coolant leak.
I have an X-Files explanation for it. I think it's possible the dead bird somehow had something to do with it, but I can't imagine how. The puddle on the ground seemed to originate from where I found the bird at the radiator's lower hose mount.
shrugs.

So somebody chime in and say, "Call us back when you find the pin-hole leak in your radiator that only manifests itself when things are cooling down or actually cold."
 

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Discussion Starter #3
Okay.... so I finally found the pin-hole leak. It was one that only manifested when things are cool or when its cold outside. I don't know how it happened, as it was on one of the vertical radiator rows -- on the inside of the radiator. I think that poorly fated bird might have somehow done it. The radiator leak explains everything else, though it is a bit uncommon to have one that doesn't show its face when hot and under pressure.
Oh well. Thanks for the help.
 

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Bad Karma! You killed the bird, and received the evil eye coolant eating ghost as a permanent guest. Until you properly honor it's death, you will be forever haunted.....

A gallon low? Wow... You are fortunately that air locks in the dual heater lines and a flicker of indication from the dash was the only hit. Keep an eye on those head gaskets.
 
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