Engine coolant inside catalytic converter

I don't know

 

Look at the coolant flow diagrams - it isn't a binary choice.

 

The coolant flow diagram show the flow of coolant if the ehr is functional. I wonder if it is a good decision to bypass a leaky EHR. In that way, bypassing a leaky EHR, the coolant flow diagram is irrelevant.

 

Long time lurker, first time doer, have some notes:

2019 Prius AWD-e, 118k miles. Bought and registered in Maine. "Should" qualify for the 150k warranty though the TSB excludes mentioning this car. As much as I'd like a new cataltytic converter as a wear item, I have little faith in my dealer's ability to diagnose this, get the part on time, etc.

Changed oil 2 weeks ago, looked around under the hood, all satisfactory.

Popped the hood last week to check the oil "once it gets some color to it", noticed overflow tank dry. Topped off.

In traffic, noticed light white smoke/steam from exhaust when nailing the gas. Uh oh. Turned around, came home, was a 35 mile round trip, overflow was dry again.

Did not notice any orange coolant in muffler tip; exhaust had moisture but looked like normal combustion byproducts/ condensation. In other words, caught this problem really early.

Obtained the NAPA 10777 copy of the Gates hose. The old hoses down by the exhaust were baked on so I cut a slit to remove the first one with a utility knife. Didn't like the mark it left on the nipple so I tried using a pliers to twist the second one off. The nipples and lines are made of thin, 0.030" or so metal and I dented the second, upper nipple. Manipulated it back to nearly straight, affixed hose, had a drip every five seconds. Drained again, dried things out best I could including a day of air drying, put Permatex Ultra black RTV on all the surfaces, reassembled, works great. (RTV loves moisture.)

My car took about a quart of coolant for the refills. There's some sort of valve lineup it does because the first few minutes of running had zero heat then suddenly it came in spades. I imagine this is when the overflow tank was consumed, refilling the heater loop, as it needed topping-off. Other than this the car burped fine. On the second cold start I had good heat within 1.5 miles so the car STILL warms up pretty fast.

I did have a code for the electric water pump overspeeding. It doesn't light the CEL. I never had a CEL, overheat, or any other dash warning throughout the experience. The pump likely oversped during the refill/ burp process.

As a temporary heat exchanger cover, I looped one of the hoses around to the other nipple and clamped it with a spring clamp. On the test drive, it blew off, as I didn't blow down the heat exchanger with shop air and the remaining coolant boiled and spontaneously created lots of pressure. Ironically, the hose that blew off blew clean off, without damaging its nipple in any way. This also says to me that my exchanger is still in good enough shape to not be able to vent this built-up pressure through its leak fast enough. One should blow down their exchanger then drive around uncapped for a few days, IMO.

Lots of talk about capping the exchanger. I wound up using Dorman #02250 plain rubber caps. Yes the exhaust gets hot. No, I don't expect that heat to conduct down the several inches of thin-wall pipe to the caps. There is no air movement, as the caps stop that. If one looks elsewhere on their exhaust, they'll notice 3/8" thick solid hangers that extend a few inches and hang from... rubber! Will report back if this plan fails.