Coolant level low

Part of what's making this interesting is that not every Prius generation has an "overflow bottle".

Gen 1 and Gen 2 have overflow bottles. Gen 3 has a "degas bottle", and so does Prius v. But Prius c still has an overflow bottle (I think). Gen 4 can come with either an overflow bottle (PNC 16470) or a degas bottle (164A2).

If the bottle has just one hose, going to a radiator filler neck just below the cap, it is an overflow bottle. It is a stagnant backwater of coolant, and when the engine heats up and coolant escapes under the radiator cap, it ends up in the bottle. When the engine cools and the coolant contracts, it gets sucked back in.

If the bottle has two or more hoses and you see coolant flowing right through it while the engine is running, it's a degas bottle. It is not a stagnant backwater but a regular part of the flowing coolant path, and the hoses are arranged so that any gas bubbles in the cooling system naturally get delivered to it (hence the "degas" in the name).

The Gen 4 degas bottles have the extra bleed cock, as shown in the photo above. The Gen 3 degas bottles don't; they have enough headspace for the bubbles you'd normally get while burping the system, and any extra gas just escapes the next time you take the cap off.

The Gen 4s that come with overflow bottles still seem to retain some aspects of the degas design; their "radiator cap" is really on a separate "filler neck" right next to the bottle, and there is an extra hose running to that, very much like the one from the system's high points in a degas-bottle arrangement. So it doesn't seem like the engineers simply time-warped back to the days of overflow bottles, but that they came up with some clever other way to do degassing.

 

Good description; they are just that.

 

Hi I'm new to posting but have read the chat a lot for my 2006 Toyota Prius. Now I have a 2017 Toyota Prius prime plug in. My issue is that I am loosing coolant. (3 gallons in 5 months) but I have no warning lights notifications or anything. Last oil change I was informed that it was critically low and that I should not drive very far but that they were out of the coolant I needed. It took approx 1 gallon to fill. I use the Verizon hum more for other reasons but it routinely tells me my coolant temperature is more than 400 degrees. Dealer says I have no warning lights so nothing is wrong. Should I stop worrying. How much coolant loss is too much.

 

Your dealer is a moron. Some fluctuation in the level is normal, but any coolant loss is too much.

The gen4 is known for having the heat exchangers in the exhaust go bad (they're used to warm the coolant faster on cold starts for lower emissions & higher mpg). The part has been often backordered lately, but if yours is bad you can probably bypass it for the time being.

That's the most likely cause of coolant losses - especially if you're not leaking anything on the ground and the engine is not running rough when it first starts.

 

I second what @ToyXW said. The first warning light on a Prius for low coolant is likely to come on AFTER the head has warped and the head gasket has blown.

Does your Prime have a lot of miles and not much EV driving? This is the first Prime I've heard of with a leak in the heat exchanger. I have heard of it in a few non-Prime Gen 4s. But that does certainly seem to be the case for you unless there's a leak in a hose, the water pump, or the tank. If that's the case, you'll see pink residue in the engine bay and maybe on the garage floor.

And for comparison, my 2017 Prime with 67k miles hasn't lost a drop.

 

I don't see coolant leak anywhere. Good to know about the lights. The car just broke 100000 miles but since I got it I mostly run ev. My Hum warning usually occurs once I get out of EV mode

 

i would double check the coolant temp, sounds like trouble

 

400 degrees is HOT HOT HOT.

It really sounds like a leak in the exhaust heat exchanger. I'd think you'd see some white smoke in the exhaust as the engine warms up. It would probably stop smoking once the engine is warm and the car stops using the exhaust heat to heat the coolant.

 

Do you have any paperwork from them saying this? It may come in useful...

 

Seriously; TAKE YOUR CAR SOMEPLACE ELSE FOR SERVICE. Perhaps another Toyota dealership; if you don't have confidence in an independent shop. Losing that much coolant in that short time frame; put you is serious jeopardy of blowing up your engine - especially since your running mainly in EV mode. When your computer asks your gas engine to switch-on it may NOT. Normal engine operating temperature should be around 200 degrees F. If one of you sensors is seeing 400 F; I would think that would trip the temperature- and CEL. I believe that this would be stored in your car's diagnostic history log - accessible by any dealership scanners.
Any shop can verify the coolant leak by doing a simple, coolant system pressure test. It should fail the test. Since your not seeing any coolant on the ground; have the shop loop-out the exhaust gas heat exchanger - then rerun the coolant system pressure test. If it passes, look into your state's law regarding catalytic converter warranty. In a CARB state, this would be covered under warranty; that exhaust gas heat exchanger is part of the smog/emissions system. 10 -yr/150K-miles here in Ca.

Hope this helps and saves you some money....

I believe someone on this forum has written a DIY on looping-out the exhaust gas heat exchanger.

 

You basically disconnect the coolant inlet and outlet hoses, cobble some sort of U-joint hose splice (say two 90 degree elbows with a few inches of coolant hose), connect both hoses to that, secure it, and maybe cap/plug the open spigots on the exhaust.

 

Wait—the coolant level is supposed to be at the low mark with a cold engine. The coolant expands and the level reaches the high mark when the engine is warm.

 

Reading the manual, the marks are for a cold engine. I still wouldn't fill it up all the way to the high mark for peace of mind against expansion-caused blowups though.

 

Check the coolant level in reservoir when hot, then cold: I suspect you’ll see little if any difference.

 

I never did on my newer cars. However, on my 1985 Corolla, the difference was huge. There was an overflow hose though, and there would be no damage if coolant overflew—albeit a toxic spill on the ground.

 

This is the density vs. temperature graph for coolant mixtures. It looks like the volume expansion of coolant between cold and warm engines is about 5%. If the coolant capacity is 8 gallons, this is almost a half-gallon difference; so, yeah, it's a huge effect regardless of what kind of car you have.

https://petra-uk.com/water-glycol-mixtures/