September 2019 Blog …. American Prius Cold Start HC System

One unusual system that the 2004 – 2009 Prius hybrids have is a heat recovery system. Another way to recapture wasted energy is to keep some hot coolant ready. To do this Denso came up with a simple but effective design. Whenever we shut off our engines and allow them to cool down, we have to heat them up again. Those in cold climates know all too well how that works! Hot coolant is stored in a stainless steel “thermos” type canister and then recycled into the cylinder head as needed, keeping the head warm and the engine ready to go into closed loop operation faster. This is North American system only. A stainless steel tank located under the left headlight has a vacuum chamber between the inside and outside of the bottle. When re-starting a Prius the hot coolant inside is pumped into the cylinder head to warm it up before the ICE fires up. This helps cut down on HC emissions on a cold start.

A three position valve with a position sensor built in is used to direct the stored hot water.


and repairing this system Tech Tip:  Disconnect the 12 v pump on the side of the storage tank when working on the cooling system so you do not get scalded as it is a passive system and it will pump coolant without warning.  The pump is round and mounted to the back of the hot water tank. Easiest way to access the two wire 12 volt pump is to turn the wheels all the way to the right and pull back on the left front fender plastic inner liner and look inside. It is right there looking back at you. Leave a shop rag hanging from that area so you don’t forget to plug it back in. If you do forget to plug it in or it tries to activate (it is a passive system) while you are working on the car it will set a code and light the MIL lamp.  Just clear it later on.

So there are some differences with hybrids vs. conventional cars but your ability to service and repair them can be learned and with more training, tools and equipment you will be able to keep up with the changes in the fleet.