How to make sure coolant is not leaking

I’d monitor it every day over the next couple weeks to see if the level drops.

Yes, a sign of a head gasket failure and coolant getting into the combustion chamber and going out the exhaust will be white smoke and a sweet smell from the exhaust pipe.

Also follow the two coolant hoses which connect to the radiator. Any signs of a crusty pink substance on those hoses? What about on the radiator fins and thermostat housing?

Is the coolant level in the radiator full? Only check when the radiator is cold.

Open the hood after you have driven for a while (engine running mode). Do you get a whiff of warmed up coolant? Could be a pinhole leak somewhere.

 

Just monitor for a while now? With our 2010 (purchased new in November 2010), there was very slow drop, and now (93K kms) it seems stable, levels haven't budged from full line, since October 2018. My maintenance log excerpts:

====
Friday, April 03, 2015
km: 51198
* changed engine oil and filter
* lubed all hinges
* checked both air filters (blew out and reinstalled, look good)
* topped up both coolant reservoirs, both were near bottom line
====
Tuesday, October 23, 2018
km: 77900~
* Topped up engine coolant reservoir, it was nearing the low line. (Inverter coolant has not budged since last top up, still at the full line)
====
Thursday, October 15, 2020
kms: 88155
* changed engine oil and filter
* changed engine and cabin air filters
* changed engine and inverter coolant
Note:
- engine coolant drain: 4.25 liters roughly
- inverter coolant drain: 1.5 liter roughly
(verify both, with note in garage)
- negligible amount drained from engine block drain, not worth doing in future
* drained Oil Catch Cans:
#1: 25~ cc (at most)
#2: nothing
====

 

does the coolant level depend on the engine temp?

 

I’d expect it to, since I can’t see the Prime being different than a full time gasoline burner in this regard. But OP mentioned the reservoir level is below the low line.

 

If monitoring of the reservoir level suggests that there is a leak, and it can’t be found just by looking, the next step would be a pressure test, as mentioned in the T-SB-0135-19 bulletin (PDF) and explained in the Repair Manual (more info) under Engine/Hybrid System: 2ZR-FXE (Cooling): Cooling System: On-Vehicle Inspection.

For this, you’d need a radiator cap tester and a suitable adapter. (Despite the name, you’d be using the tool to pressurize the cooling system, not to test the reservoir cap.) The tools listed in various editions of the Repair Manual (Toyota 09230-40010 or BANZAI RCT-2A or IYASAKA RCT-3 tester and Toyota 09231-40030 or BANZAI RCG-3TC or IYASAKA IRC-3TC adapter set) may be difficult to find. Instead, the CTA Tools 7060 tester and 7103 adapter, Mityvac MV4529 tester and MVA111 adapter, or OEMTOOLS 27069 tester and 27248R adapter, for example, should work with the 34 mm diameter and 3 mm thread size used on Prius and Prius Prime cars.

 

And don't worry too much during that week.
It is not unusual for that to happen early in the winter if you check it COLD........on any vehicle.

 

I live in Canada and am also experiencing the same issue on my 2017. Car now has 140k on it.

First I'd suggest an oil analysis. I sent some oil off for analysis and coolant isn't ending up in the oil, so not a head gasket issue in my situation. For piece of mind you might do the same.

It's quite a slow leak over a number of many weeks before the level goes from high to low.

No idea where the leak is but every once in awhile I can smell coolant..the smell of coolant on a hot component.

Let me know what you find out.

 

Not sure that's definitive. If coolant continues to drop, try leak-down test? Also boroscope inspection of the cylinders.

 

I disagree.

Having done these tests on multiple cars over the years, coolant (and any other contaminant) will show in the oil if it's making it's way there.

One year there was about a tablespoon of water detected in my oil on another car. Oil capacity was 10L. This was deemed condensation after the car was sitting in storage all winter.

So these tests are quite sensitive to any contaminants.

I'm confident if any coolant was in the oil, it would be detected...especially so since I told them to look for coolant as a contaminant.

 

I had the low coolant issue and had it repaired per the TSB, no charge. The problem was with the exhaust heat exchanger.

 

Coolant doesn't always leak into the oil.

 

And if it's making its way into the exhaust heat exchanger or the combustion chamber, it will not show up in the oil. But, unlike leakage into the oil passages, it will create the smell you mentioned.

 

That's going to vary. It's dependent on how much ICE time you drive on a daily basis. If your running in EV mode all the time, you probably won't see any coolant loss until you take a long trip and run the ICE, or use your windscreen defroster mode - that'll fire-up the ICE too.
Just monitor it, based on your ICE usage for now. If that's your first top-up in 4 years, your doing great.

 

Yes, the level would continuously drop and I had to keep topping it off until I had the repair done. It took a few weeks before the replacement parts came in (two gaskets and heat exchanger.)

 

I specifically mentioned head gasket.

If it's a head gasket issue coolant will typically contaminate the oil.

 

I bought my 2017 touring four in March with 22,500 miles. I noticed the coolant was low shortly after getting it home. It was bought "certified" and should have been caught by the dealer. Anyway I topped it off and have monitored it since with no noticeable loss. My guess it just lost a little over time or might have came from the factory that way. The plastic resevoir on mine is very difficult see through and I notice yours is as well.

 

Or not ....

 

The picture above explains it all. Most head-gasket failures into a coolant channel results in "boiling coolant" and engine over-heat. It's much easier to push exhaust gases into the coolant channel, than to pull coolant into the combustion chamber - pressure & density differential. With today's modern, lean-burn, ECU driven engines - a CEL would've been throw before it could suck coolant in the combustion chamber. Bottom-line, you would've saw this coming.