Turn the rear heater to "Auto" and push both slider controls fully to the rear - selecting air from only the floor vents and hottest temperature (in fact this works even if you only select moderately warm temperature).

Have the rear heating switched on when you drive away from cold. Initially the rear blower does not run but within the first km or 2 mins the rear heating will start up all on its own, blowing warm air, well before the front system gets warm. It suggest two points


1. That sensors/"computer" disable the rear blower when heat is demanded but the coolant is cold - avoids unknowingly freezing polite Japanese passengers. Very nice, thank you Toyota.

2. More important for us, that the rear heater matrix is the priority route for heated coolant from the cylinder head. We suspect this is how warm coolant gets down to open the thermostat which is why there is no coolant shut-off valve for the rear heater. What's more, if the rear heater matrix is clogged you not only lose comfort but there is a serious risk that the head has to get significantly hotter before the thermostat gets the message that it's time to open.

So ... might measuring the time for the rear blower to start under these conditions be a useful test to eliminate a possible source of overheating?

- (#6709) Dave Mason, 9 Jan 03

Well, to start with the test only applies to Royal Lounges with climate control...
The rear heater is recirculate only, so you'll feel warm air faster than from the front heater (warmer output for same deltaT.)
Remember also that the rear matrix is lower in the car and thus has less head for the water pump to overcome.

I think your logic re the matrix as a bypass might be back to front- the return water from the matrix is COLDER than the water in the block. This is injected right at the sensing end of the stat. Any other 2C uses the feed to the injection pump from the rear of the head as the bypass- quite sufficient to ensure flow around the block/ head. Plus we have the same water pump as any other 2C, but twice the heater flow (matrices are in parallel), thus the absolute flow through the rad even with the stat fully open will be lower in the townie than in say a Corolla.
Remember that we don't blow headgaskets on warmup, rather after a hard uphill thrash with a dodgy rad cap...

- (#6710) david miller, 10 Jan 03