No Heat: Coolant Pumps During Main Mode, Not While Running

Post 7 above is straight out of the repair manual and includes tests using a bidirectional scanner your hybrid shop should have. Complete guides are available for a fee from the Toyota “Technical Information System” (TIS link below) although this system is oriented to techs with some Toyota training.

Your shop could verify coolant flow through the heater is not blocked inside the heater core “radiator” or by other devices in the same flow path.

So one can verify heater flow (not just coolant in the hoses) and manually actuate the “air mix control damper” (blend door) with a scanner.

Detail from post 7 with Air Mix Control Damper highlighted in Yellow



Accessing the damper is a multi day job requiring removal of the dash and hvac system followed by reassembly. Not the recommended diagnostic method.

Toyota Technical Information System (TIS)
https://techinfo.toyota.com/techInfoPortal/appmanager/t3/ti?_nfpb=true&_pageLabel=ti_home_page&goto=https://techinfo.toyota.com:443/agent/custom-login-response?state=7P4ca2qKIFPfikQ6Zr8V18CVeOk&original_request_url=https://techinfo.toyota.com:443/

 

That does sound like the blend door. You can do some diagnosis with Techstream or another scan tool that can show live data, and watch what it shows for the air mix door position as you change the settings.

Sometimes there's an electrical problem with the servo (or with the driver chip, which, in gen 3, is part of the small wire harness that drapes over the heater and connects the three servos to the amplifier; the three plugs that plug into the servos have driver chips in them). Sometimes it's a mechanical problem, a gear or shaft popped out of position.

 

I would certainly verify there is water pump forced flow in and out of the heater before tearing into the dash. There can be several blockage points in the heater flowpath.

 

I am not sure how AC operation and cooling proves anything if the blend door is blocking airflow through the heater core.

Second the measured hose temps may all be all too low especially if the engine is fully warmed up to 190f operating temperature.

I have measured part of the heater coolant flow circuit at the drivers side rear temperature sensor using an infrared gun. I found it to be within 3 degrees of the scanner reported coolant temp.


You need to put the car in
Maintenance Mode so the engine continuously runs. This is the only time the water pump is guaranteed to run.

Ideally watch the coolant temp and the water pump rpm with a scanner and then place the hvac system into heat. I would expect a hotter inlet than outlet.

With no flow the heater hose temps would be lower than engine temp.

If there is no flow through the heater hoses it could be the heater core. . Or it could be blocked by the egr cooler. Or by the thermostat bypass inlet.

In Techstream Maintenance Mode is called Inspection Mode and can be directly turned on. Otherwise,,,


Maintenance Mode
Continuously Idle Engine

Press Start button 2 times without Brake (Ignition On)
Press Gas Pedal 2 times
Press Brake and shift to Neutral

Press Gas Pedal 2 times
Press Park
Press Gas Pedal 2 times (hybrid warning light will start flashing)

Press Brake and Start button (starts engine, don't drive it, shutdown to return to normal)

 

I went out and traced the heater hoses on our v and agree the engine to heater hose is on the bottom rather than the top.

Perhaps inlet and outlet meanings were lost in translation or were different on similar Corollas with the 1.8L..

 

That does sound like a flow blockage but it still could be at the egr cooler, thermostat or connecting hose. The heater has smaller passages that could be blocked but didn’t you say someone flowed water through it successfully?

I would wonder if he somehow blocked the thermostat bypass or has pinched the hose shut going to it. That hose goes from the egr cooler along the front of the engine to the thermostat. It is obscured from view by the air intake plastic housings.

————

Important info: Flow through the heater should continue if the heat is on or not.

The thermostat bypass should be flowing if the thermostat is open or not.

The bigger engine to radiator hose flows do get mostly blocked by the thermostat when cold.

So there is a coolant flowpath that is open all the time through the heater and one that is controlled by the thermostat.

Flow is different from coolant pressure. You could have pressure without flow, sort of like a water faucet.

Besides something blocked in the heater flow path, you could have a pinched off hose or some hose on wrong.

I will try to get a sketch of the flowpaths for your mechanic.

 

Here is a quite amusing video that actually has some good info on the subject.

Update: This is a gen4 engine to gen3 car swap which are known to have overheating issues.

In another short video he captures the “heater actuator” air mix blend door in action.

Update: After reviewing comments on this 5• year video, it became clear his problem came after a gen4 engine swap into a gen3 Prius. These engines have different cooling connections and flowpaths. In these cases, air locking on the heating flowpath is more common.

About a year later he commented about modifying the gen4/3 thermostat “jiggle pin” by drilling it out.


Realize this fixed his gen4/3 heater problem. However many other viewers commented how sealers had clogged their heater core.

 

^ PriusChat alumni @Ragingfit.

 

On the web, you can see a lot of spectacularly ill-informed uses of an IR temp gun. I saw this in a YouTube video once:



The guy thought he really was measuring the temp of the narrow sliver of exhaust pipe down between the heat shield and flange where he had put the laser spot.

The tool quite simply doesn't do anything like that at all. It shows you an average temperature over a whole round area whose size depends on the distance from the gun. There should be a chart or drawing like this in its manual somewhere:



(I cribbed that one from the manual for the exact model of gun being used in that YouTube video ... which the person making the video must never have looked at.) The laser spot is nothing but a rough guide to where the center of the whole measured area is. Some tools have wider or narrower spot size-to-distance ratios, so be sure you know what it is for the one you're using.

Also, there's the matter of measuring temps off surfaces with different emissivities.

The tools are handy and fun, but very easy to get fully bogus results with, if not used with enough planning and care.

One can get clamp-on thermocouples where HVAC service tools are sold, and those eliminate a lot of the slop that comes with an IR gun measurement. (Some IR guns will even have a socket on the side where you can plug a thermocouple in, and then the gun will display the accurate temperature from that.)