2019 Prius Slow Coolant Leak

Sounds like just the ac and fans were running at first then the rattle on engine startup.

But you are losing a lot of coolant in one day. If it was coming out at the exhaust heat exchanger I would expect some steam soon after the exhaust heated up.

With that much coolant loss the combustion leak detector test might be an easy test without having to remove the windshield wiper assembly to access plugs.



Any shop could do that test as well.

If it is negative, you could do the exhaust heat bypass or move straight to a borescope test with the specific setup as described in item 2 of post 16 and as demonstrated in the videos.

 

What was the engine coolant temperature during the trip?

Do NOT use the stop leak. That is something you use as a last resort

 

just before you sell the car

 

Are these the same people that claimed you passed the coolant leak test???? Why would they suggest you place stop-leak into a system that they declared; there was NO leaks? I've never heard of a "professional" shop suggest putting stop leak into a cooling system - sorry, that's usually the purview of a shade tree mechanic or used car lot. Cover-up the symptoms so they can make it someone else's problem.

I would find another shop; preferably a Prius specialty shop or an old mechanic that knows how to test and find problems - rather than just point a parts cannon at cars.

Good Luck......

FWIW; metal expands when heated; so their theory that the leak is caused by heat - doesn't really hold water. Historically, on Prius coolant leaks happen when cold, because the aluminum block contracts and separates from the head gasket. As the engine warms up, the aluminum expands and closes the leak.

Easy to prove; do a coolant leak test when the engine is at operating temperature. Simple - NO GUESSING. I still think they messed-up the initial coolant leak test or just didn't do it properly - because your coolant is going some place and that's the fact.

 

Keep in mind most service advisors have never been mechanics. Most dealerships might have one Master Mechanic and he may not have performed your borescope test OR the exhaust heat exchanger is leaking again.

This should be the last resort primarily due to its potential to clog up heater and radiator heat exchangers AND the difficulty to actually get it out, even right away. The term flush is used loosely, the standard coolant change is a drain and fill which leaves almost half of the coolant in the system. The coolant system also has multiple parallel paths which makes a real flush difficult.

This is a good idea if you do it quickly. You should not be piling up 200 miles a day with this issue. If you are not in engine replacement mode yet you could soon be there.

In defense of the service advisor, stop leak can be diagnostic of a hg leak if it stops the coolant loss. But if I were his defense lawyer in court and the prosecution had an expert witness, I would lose.

 

A hg leak is rare on a gen4 but could happen after a serious overheating event.

The thermostat must be open and coolant circulating with the engine running during a combustion gas test. That requires full operating temperature first and perhaps switching to maintenance mode second for continuous engine operation.

Even then, a hg leak may seal up at operating temperatures. Your coolant loss seems higher than a normal early onset hg leak.

It passed the borescope test. No obvious leaks. It points at the exhaust heat exchanger again. I would bypass it. Again. If a bypass fixed it I would leave it bypassed and force the exhaust heat mechanical thermostat into its extended, warmed up state.




Gen4 Coolant Flow



Exhaust Heat Control Valve Forced Open on a Gen3


Finally, a borescope test can provide a false negative if done by an inexperienced person. A exhaust heat bypass that does not solve the issue points right back to the hg.

 

Many have been frustrated by assumptions that a new part can't be the issue. If nothing else a bypass is an easy diagnostic.

Again, no obvious releases points to a exhaust heat exchanger or a hg, both of which vaporize coolant out of the exhaust.

A borescope test looks for "steamed cleaned" pistons, sometimes a clean spark-plug like you show in post 1 and, sometimes coolant in the cylinder or a drip from the head gasket. Seeing it drip takes a quality articulating borescope on a cold engine with a manually pressurized cooling system. Often requiring the car to be left overnight and not started. Room for error.

 

I would also monitor the coolant temperature in real time as noted before using a simple bluetooth scanner and a phone app or a dedicated obd2 display.

 

You should be monitoring the coolant temperature while driving. That is the only way to rule out an intermediate water pump. I thought you were going to do that.

 

Use your app while you are driving to verify the engine stays below 210f with typical engine on temperatures around 195f or less.

Also note in post 31 there are two "engine" temperature sensors. My gen3 experience is the engine temperature is a bit higher than the exhaust heat exchanger when the engine coolant is warmed up.

This "Car Scanner" app trending capture shows full load peaks when driving up a steep grade several times with ac on in the summer.


Each horizontal axis time period is 8 minutes

Note how much the engine cooled in the last segment. That period was Ready engine off with the ac on and represents the rapid thermal cycling a Prius can experience. It was followed by one more high load hill ascent.