Cat Stolen on my 2015 Prius, but aftermarket Cat doesn't have coolant line?

In theory you can just connect those two lines together and everything will be fine. You'll only have slightly less MPG on cold starts for lack of this heating feature.

But in practice there's probably a smarter place to cut and connect these lines with better than normal hardware then just guessing DIY style. Perhaps stop by some local muffler shops and inquire about their method? Or perhaps someone on here has already worked on this challenge and they'll post soon?

 

No you're just putting those two lines together to complete the circuit of the coolant If you were to do this in anything else like an air conditioning system or another water system and you remove a component that liquid or coolant flows through and you're going to eliminate that component you have to make that coolant still get from point A hose to the other b hose usually a barb fitting and two clamps and if you trim the hose back enough you can pin it against the firewall or against other components that don't move to hold it up with panduit straps or zip ties as most people call them and then you're golden forever you lose a couple miles to the gallon for a couple minutes at best while the car preheats so when you're New York winners you'll need to get in the car and get them moving right smartly so the car will get heat it will take 8 minutes longer or something..

 

maybe he forgot to clamp the hose

 

Yeah, shouldn't be too hard to track down a leak like that.

Just to stir the pot a little, it's not really strictly necessary to connect those two lines together down there. You could also consider just plugging them. The reason has to do with the way all the coolant lines are routed.

It's not super easy to see in this diagram, but the double-pipe thing 87208 is what carries the coolant (on its way back from the cabin heater core in hose 87245A) down to the EHRS (through hose 87245C), and the heated-up coolant coming back from EHRS (hose 87246C) comes back up the other pipe of the double-pipe thing. From there it passes through hoses 87246B and A, and joins the coolant flow at the forward end (exit) of the EGR cooler (25601L), returning to the water inlet nipple (the one that bypasses the thermostat).

But if you look again at that double-pipe thing, you see another nipple that tees off from it with the coolant on the way down. That coolant, instead of going through EHRS, just takes the hose 87245B and goes straight to the EGR cooler (25601L again!) by the nipple at its aft (inlet) end. This is the coolant flow that actually cools the EGR (which is why it isn't sent down to the EHRS to get hotter first). After passing through the cooler it rejoins the other path on its way to the water inlet thermostat bypass.

So if you just plugged the two EHRS ends of that double pipe down under the car, you would not be blocking any coolant flow totally off. It would just all be forced to take the other path through 87245B and the EGR cooler.

Finally, if you compare the two options (join those two ends to each other with a hose, as most people are doing, or just stopper them off so all the flow is through the EGR cooler), you'll realize neither option is really quite right.

As designed, the system has two parallel coolant paths there, one through the EHRS and one through the EGR cooler, which means the flow is proportioned between them according to the flow resistance of each path.

If you join the two ends with just a short hose (very low flow resistance), overall flow in the total circuit will be increased, but it'll mostly be going through your bypass hose, and you'll have stolen a bit of flow from the EGR cooler.

If you just plug those two ends, so the only path left is through the EGR cooler, it will be getting plenty of (all the) flow, but the total flow through the circuit will be reduced some.

Because the cabin heater core is the first part of that circuit, you might notice slightly better flow through the heater in the join-em-with-a-hose case, or slightly lackluster heating in the just-plug-em case.

The fix that would make an engineer smile would be to connect those two ends with a hose and a bit of restriction, chosen to have the same flow resistance as the original EHRS exchanger. Then you will end up with the same overall flow through the cabin heater, and the same portion going through the EGR cooler.

But it might not be necessary to worry about it that much. I don't think any of the other folks in your situation have.

 

I was going to say just use a flow restrictor or a smaller barbed fitting inserted in between the two hoses like a restrictor plate and a thermostat of a race car but now we're going to do that with a barbed fitting so you slow down the water going through the restrictor creating near the same resistance path as it would have when all the nonsense was plugged up like the factory yep pretty spiffy. I used to use restrictor plates and high performance street cars long time ago and like in the '60s and '70s when lots of folks didn't understand certain things about thermodynamics and what have you.

 

Really sucks that a lowlife stole your cat. Did you have a cat shield on it or some other type of protection?