Engine Coolant Is Slowly Disapearing

How many miles on the car? One of the more common issues with these 2010 cars, is unfortunately a head gasket failure. Which starts with losing coolant

 

How many miles are on the odometer?

Does the engine shake when started up after sitting for a while?

Any error codes such as cylinder misfires?

Many high mileage 2010 Prius cars develop a head gasket leak that allows engine coolant to enter the cylinder(s). It is burned off during normal operation, but the coolant can accumulate while it sits between startups, causing bad shaking until the fluid can be burned off.



Posted via the PriusChat mobile app.

 

Odds are extremely good its slowly leaking coolant into a cylinder and combustion gas is causing the air bubbles. Six months and overheating is a bad situation that likely will result in a need for a replacement engine.

When a head gasket on a Gen3 Prius fails it often causes severe rattling occasionally on cold starts that clears in a minute and then runs fine. Later it can be minor vibrations everytime the stop start engine starts while driving. Eventually white smoke can be seen at the tailpipe as the driver revs the engine while parked.

Pressure testing the cooling system won't show a head gasket leak on these engines. Normally a compression test is useless as well. The best test is a borescope inspection of the cylinders which is likely to find an abnormally steam cleaned piston or sometimes even coolant.

 

Cylinder leakdown test, not compression test.

Posted via the PriusChat mobile app.

 

Just check no 2 cylinder very well . If you have a boroscope or a way to see in the hole look at the top of the piston real good for small cracks maybe a hole it's not really uncommon to find this I had a mechanic put it to ZZ back together with a fracture in the top of a piston complete waste of time

SM-A715F ?

 

In 2017 Toyota issued a "Warranty Enhancement Program", about "rough idle and/or tapping noise from the HV transaxle at cold start" due to a sticking EGR valve due to excessive carbon build-up". That's quite a run-on sentence, and no mention of head gasket failure.

And, they toss in "while the majority of vehicles will not experience this condition", yeah right...

200K you're well overdue for Exhaust Gas Recirculation carbon cleanout (including the EGR passages in intake manifold). Sadlly, Toyota doesn't alert owners to thisl doesn't acknowledge the connection.

More info in my signature.

 

It might be fair to say "rough idle and/or tapping noise from the HV transaxle at cold start" can be a Prius mystery with more causes than have been successfully pinned down.

I lived with it in my Gen 1 from 198,000 to 230,000 miles (then presumably the person I sold it to lived with it). Because that was four years of driving and 30+ thousand miles, it's unlikely to have been a head gasket issue. And Gen 1 had no EGR system.

Of course in that case there wasn't slowly-disappearing coolant either.

 

Who's "them"? Did they clean the EGR passages in the intake manifold as well? Were they forthcoming with what they did exactly, or not so much?

Bottom line, dealerships in particular are very reluctant to do "cleaning", just like slamming fresh (and full retail priced) parts in. DIY is pretty much the only way to get it done thoroughly. And cheap as dirt.