No Heat After Multiple Tries? Gen2 07

In my experience, the 700 to 800 watts of electrical PTC heat before the engine warms up is humanly imperceptible. The only way I can tell if it is on or off is the difference in how fast a foggy windshield clears. It takes more like the 5300 watts worth of hot coolant you get from the engine to make the vent air feel actually warm.

(* I got the 5300 figure from gen 3, but assume it's the same ballpark for gen 2.)

 

That's not the only way of reading those observations.

The heater core is a heat exchanger: the liquid gets pumped in warmer and leaves cooler, while the air gets blown in cooler and leaves warmer. The gozinta hose feeling warmer than the gozouta hose is pretty much what you should expect.

That the gozinta hose feels hot is a pretty good sign the flow isn't completely blocked. If the flow were blocked, there would not be hot liquid flowing in that hose to make it feel hot.

Now, whether the flow is full or sorta reduced, that could be a question.

How much cooler the gozouta should be compared to the gozinta depends on a lot. A high cabin fan speed will take more heat out of the liquid than a low fan speed. Colder intake air will take more heat out of the liquid than warmer air. Slower-flowing liquid will come out having lost more heat than faster-flowing liquid.

So there are a lot of questions you can ask or answer about the job that heat exchanger is doing, but good answers kinda call for good measurements of the gozinta and gozouta temperatures, and the fan airflow, and air intake and discharge temperatures. And the next challenge is the Toyota docs don't really tell you much about how to do that.

I did just now look in the 2004 New Car Features manual and they did give the 5300 watt figure for the heat exchanger capacity for that generation. That presumably applies when the coolant is at full operating temperature, thermostat open, and the fan blowing the maximum airflow. If you knew the design flow rate of the liquid through the core, you could work back from the 5300 watts and get the temperature drop from gozinta to gozouta. If you knew the design airflow at max fan, you could likewise work out the expected air temp rise at 5300 watts. You could compare those figures to the temp drop / temp rise measurements you have IRL and get a good idea what's going on.

Trouble is, the manual doesn't give those flow rates. It might not be hard to measure the airflow using the tools home HVAC techs use. The liquid flow rate you could get by inserting a flow sensor inline with the core—assuming you had a known working-right Prius to do that in, and not one where you already wonder if the flow rate is right.

 

You could do that but then it will throw codes because that would also disable the thermos mechanism.

 

Wait, since when do you get P1121? I don't see that in this thread except for your last post.

Inspect the connectors on the plug to the valve for corrosion. A good valve won't work if it cannot talk properly with the rest of the car. If you have Techstream use it to test the coolant control valve.

 

Clear it and see if it comes back.

Happy the heat is working now. Crossing fingers it stays that way.