Please tell me it might not be head gasket

It's better to remove the O-ring from the coolant bottle cap. That'll prevent the cooling system from pressurizing and limit the amount of coolant being dumped into the combustion chamber.

ANY sealer you put into your cooling system is going to clog it - so even if you do fix the head gasket, your cooling system will be compromised and will need replacement. Quick fix NOW for expensive repairs later; if it works and it's ONLY temporary.

Your car, your choice.....

 

Likely the head was machined, not the block....

 

Most likely it is the head gasket. I had a mechanic keep wanting to verify it was the head gasket, supposedly to save me some money. It passed a pressure test, and he said he didn't see any coolant or "steam cleaning" in the cylinder. So I'd take the car back and it'd burn off more coolant...slowly. Brought it back. Still passed a pressure test. Inspect the system for leaks, no leaks. Coolant is still going somewhere.

I eventually just asked him what other explanation there could be on a car known for having a head gasket failure. When the motor came apart, they said it was failing in a spot, but just hadn't fully gone yet. So slow leak, only had the "death rattle" a few times, and one CEL for a misfire.

 

You have to pressurize the coolant system when cold and use an articulating borescope.

Gen3 Prii are not the only engines with slow cold start leaks.

The Ford Edge Ecoboost engines are very similar but with a diagnostic advantage. Their misfire detection accurately determines cylinder and they provide a PO316 "Engine Misfire Detected on Startup (First 1000 Revolutions)" that the Prius does not generate.



Ford also does not recommend hg change alone. Instead they sell a discounted engine. Toyota recommends rebuilt but does not discount engines or short blocks.

Toyota did acknowledge their low tension ring problem and replaced thousands with excessive oil burning and resulting carbon buildup free in the first 5 years or 60,000 miles.

 

Yep, the mechanic would've found it; if the engine was cold and he pressurized the coolant tank. Gotta remember that gases can squeeze into places where liquid can't. ie breathable Gore-Tex. The times you had the death rattle, coolant actually got into the combustion chamber. If you monitored you engine operating temperature; you would've found your engine was running hot too; due to hot exhaust gases being pushed into your coolant system. The chemical coolant test may show that, if your leak was large enough.

 

I'd like to be a fly on the wall, when they discussed that 60k cutoff.

 

It's very doubtful the test for the coolant will work, on a Prius.
It doesn't take a lot of coolant, you may not even notice it going down for a long time.
As was stated a few times, pressure test, bore scope.

Mine didn't have the steam cleaned pistons, because I caught it as it first started.
And changed the gasket within about 500 miles of the first knock.