I've tried to research this myself, but so far all I can find are sketchy explanations of how this works. I'm not sure how it's possible for the cooling system to extract heat from the exhaust system without sometimes overheating the coolant. Over an extended period of driving with the gas engine running, there would be a significant amount of heat built up in the exhaust system, so what protects the cooling system?
I think if you check out Bob Wilson's post's. He has a neat picture of the system of the actual engine, with the system exposed so you can see how it hook's up and returns to engine and exhaust.
There's some valve that shuts off coolant flow once temp has climbed sufficiently. At least it's supposed to. Someone here posted in the last few days about that valve failing, and an ensuing overheat issue?
Excerpt from Repair Manual. Seem there's a mechanical, temperature sensitive actuator, and electronic monitoring. Also an override to prevent to much back pressure during heavy acceleration with cold engine. Exhaust Heat Recirculation System Circuit DESCRIPTION In the exhaust heat recirculation system, coolant is warmed up using conventionally wasted exhaust gas heat to accelerate engine warm-up time, enhancing fuel efficiency and heater performance. The heat recirculator is positioned in the front exhaust pipe assembly after the catalyst. Coolant from the engine flows around the heat recirculator and then returns to the engine. If the engine is started while the engine is cold, the exhaust pipe gas control actuator rod is contracted and the exhaust flow control valve is closed, routing the exhaust gas around the heat recirculator to warm up the coolant. After the engine coolant temperature rises and the engine has warmed up, the heat of the coolant expands the thermostat and the exhaust pipe gas control actuator rod extends. This opens the exhaust flow control valve to switch to the normal exhaust gas path. The exhaust flow control valve can also be opened by exhaust gas pressure to prevent insufficient acceleration performance when the engine is cold. In addition, to monitor the engine coolant temperature, a engine coolant temperature sensor (for exhaust heat recirculation system) is provided between the engine and the heat recirculator. The engine coolant temperature sensor (for exhaust heat recirculation system) has a built-in thermistor with a resistance that varies according to the temperature of the engine coolant. When the engine coolant temperature becomes low, the resistance of the thermistor increases. When the temperature becomes high, the resistance drops. These variations in resistance are transmitted to the No. 3 meter circuit plate as voltage changes. If the engine coolant temperature is excessively high (overheating), the water temperature indicator light in the No. 3 meter circuit plate illuminate to inform the driver of the malfunction.
One of the few problems I have had with the Prius was a stick puncture of the rubber inlet pipe to the exhaust cooler.
We have to be careful with names in Gen 3 (and 4) because there are two different things we could mean by "exhaust cooler". Each one is a heat exchanger with coolant and exhaust flowing through it. One is under the hood next to the EGR valve, and one is under the car behind the catalytic converters. The one next to the EGR is there to cool the exhaust. The one after the catalytic converters is there to heat the coolant.
Just wanted to say that EGR has been on vehicles (at least gasoline-burning vehicles) since the 1960s. The EGR was among the first devices used to lessen air pollutant emissions -- they predate catalytic converters by about a decade.
Probably a good place to repeat that this thread is about the Exhaust Heat Recirculation System, which appears in Prius Gen 3, and which is not the EGR system. Gen 3 Prius also has EGR, but that is a different system. The car has two different exhaust-to-coolant heat exchangers. One is for the EGR, one is for the EHRS. This thread is about that second one.
I see one person had a hose to the coolant heater punctured by a stick, I had the unit itself dented by some other road hazard which dent somehow disabled it. There were, last Friday, 497 other folks ahead of me in the Toyota "back order line" on this unavailable part for the Gen3 years. Local dealer here says the part alone will cost 2700 dollars whenever it comes in, found another dealer in bigger city selling it for 1600 dollars, whenever it comes in. Until I get the part replaced I have to piddle around town at low speeds/short drives and check coolant before driving. The dented part has two coolant lines, no one seems to have a clue as to whether or not the hoses can safely be blocked off or just connected to remove the leaking part. Lucky I have another vehicle, many folks with jobs don't. Seems there was another type of coolant heater 05 thru 09, there is a video on youtube of a fellow showing the coolant coming through the exhaust from a Gen2 because of this. Not as bad as a head gasket but certainly not good. Does not help as the cat/converter thieves simply cut these out when they strike.
