Our 2011 Prius (185.,000 miles) is running through coolant quickly. It's not the head gasket, so that would seem to indicate a leak in the heat exchanger. Replacing it is prohibitively expensive. I recall, in the old days, if you had a leaky radiator you could add powdered-metal Stop-Leak to the coolant and sometimes plug the leak that way. I don't know anything about the design of the heat exchanger. Would something like Stop-Leak fix most of the leaks you would find in the exchanger? Has anyone tried that? It seems like a cheap way to attempt to fix the problem, and if it doesn't work the bypass option wouldn't be that much more expensive. Of course, if someone was to steal the car's cat converter assy then insurance would cover the replacement. Thanks! George
IMHO, I would NOT use any stop-leak products in modern automobiles. As our ability to manufacturer finer and more precise parts, cooling system heat exchangers passages has gotten much smaller and narrower. The product you mentioned hasn't changed in four decades and is more likely to clog your cooling system, causing an overheat issue in the warmer parts of the year. The bypass is your best bet and hopefully it'll pass smog that way. Good Luck.....
I memory serves, the heat exchanger has a thermostatically controlled bypass to prevent the coolant boiling out. It it possible that this has malfunctioned?
You should do a pressure test to find the leak. Then you can know for sure where it is leaking. NEVER USE THE ANY STOP LEAK OR HEAD GASKET SEALER!!! It WILL ruin all the coolant system, including thermostat and coolant pump.
The Gen 3 Prius has had very few problems with the exhaust gas heat recovery system. It is a big problem only on Gen 4 Prius. I would be very surprised if this is your problem. Has the high coolant temperature warning light ever come on, even for a few seconds, while driving up a hill or at highway speeds? My guess is that the engine water pump is starting to fail, and there is not enough coolant flowing through the engine. When this occurs, the engine overheats and coolant is forced out the overflow on the plastic reservoir. Look very closely around the reservoir for evidence of dried coolant. You should purchase an OBD2 Bluetooth dongle and use an app on your phone to monitor the engine coolant temperature. Or you could purchase a display unit that plugs into the OBD2 port to monitor the coolant temperature. I use a P10 display. Normally, the coolant temperature will not go higher than 202F. Do a search on this Gen 3 forum about engine water pump going bad.
How certain are you? Who did the diagnosis? What method(s)? Head gasket failure by your miles is VERY common.
