Coolant going down - no idea of cause

Inspection / Maintenance Mode
Continuously Run Engine

Press Start button 2 times without Brake (ignition On)

Press Gas Pedal 2 times
Press brake and shift to Neutral

Press Gas Pedal 2 times
Press Park

Press Gas Pedal 2 times
(hybrid warning light will start flashing or Maintenance Mode Message)

Press Brake and Start button
(starts engine, don't drive it, shutdown to return to normal)

Cliff Notes:
Ignition Mode, 2Gas, Neutral, 2Gas, Park, 2Gas, Start Engine

 

Yes it does, but then he would need someone holding the brake pedal/emergency brake to keep the car from moving while another person checks the exhaust.

 

Ah yeah, that could get dangerous.

 

You beat me to it.

Googled this:

priuschat gen 4 exhaust heat recovery bypass

Then settled on this:

4th Gen DIY heat exchanger bypass solution | PriusChat

For sure it'd be good to cap the pipes. For starters, there'll be a slight amount of exhaust escaping through them?

Get coolant hose clamps, with a suitable clamping range. For example: if you push the hose on and measure OD to be 20 mm, look for hose clamps with a range something like 18~22 mm. I'm not sure they list them thus though, I have been bitten. If you for example see one that says 20 mm diameter, that's probably it's at-rest ID, and it won't be tight enough.

 

IMHO; Loop-in/cap the other end too. In case you fail emissions testing and have to re-establish the setup. That will keep you from contaminating your coolant, if you have to do that. Otherwise road debris will get in there and a small exhaust leak.

 

I would make sure the "Exhaust Gas Control Valve" was mechanically secured in its activated (green arrows) position to reduce exhaust back pressure through this system.


Red is exhaust flow when cold
Green is flow when coolant is hot


When coolant is cold, exhaust flow is directed through the Heat Exchanger by the manual thermostatically controlled "Exhaust Flow Control Valve" below.

This exhaust divergence is only for a few minutes until the engine warms up.

However after the coolant bypass "fix" the Control Valve stays in its cold position because coolant is no longer in its circuit.

 

where does your mech think the coolant is going?

 

sounds like he doesn't understand the prius system.

 

Just continue to monitor. It might be just a gradual drop over the years, you've topped it up, and it'll be stable now. Takes time.

With our '10 I noticed both reservoirs down to the bottom line after about 5 years of ownership and 50k kms. Then again around 8 years and 80k, just the engine coolant was once again down to the bottom line, needed another top up. Did a coolant change on both at 10 years and been stable since. Maybe a few air pockets from the factory, working their way out.

 

almost empty reservoir is the key here, i think

 

First, you have a 2016 hatchback and not a 2016 gen3 Prius v correct? Big differences in the engines and coolant systems.

Based on your early posts the reservoir has almost completely emptied at least twice in the last months. This is bad.

So when your mechanic says it held pressure for 30 minutes he is either not testing properly or you have a runtime leak.

About the only runtime leak that comes to mind is a head gasket leak.

But the simple problem on a 2016 gen4 hatchback is the exhaust heat recirc system. A bypass is also a low cost diagnostic. If the problem goes away you know the issue. Low cost and fast.

Assuming you have a good Prius mechanic he should know the gen4 coolant system.