What am I missing? Head gasket Dx

it's a super tiny leak that's difficult to view with even a really good camera... But the rough running and coolant loss is enough of a symptom for this known problem. Also without knowing the fixes the mechanic tried, it would be difficult to figure out what they meant by that.

 

If the mechanic did indeed do the head gasket job - the leak would be visually apparent on either side of the head gasket for confirmation. If it isn't, the problem wasn't the head gasket and he misdiagnosed it. You think the mechanic would take that hit and work for free???

Just my two cents...

 

The head gasket used is a Multi-Layer Steel gasket with a black coating. They depend on very flat machined surfaces. Severe and prolonged overheating can cause a head gasket fail on a gen3 or any modern aluminum engine. Severe overheating typically causes a warped head.

However the "normal" gen3 Prius head gasket leak does not require overheating. Most of them never overheat before early onset hg symptoms. The symptoms are severe cold start rattles that clear quickly and may not repeat for weeks or months. They pass combustion gas tests and generally produce no codes since it runs fine after the first few seconds. Mechanics are often confounded by these symptoms and suggest a thousand dollars worth of plugs, coils and injectors, usually a piece at a time. With no success.

Typically the steel hg black coating has eroded between the cylinder and a coolant passage. Acidic coolant and carbon based pinging is the most likely cause along with poor gen3 head geometry and a minimal amount of black coating.

With the gen4 1.8L engine Toyota updated the head gasket, head geometry, and rings to reduce oil burning (another common issue), along with improved cylinder cooling, egr design and more.

What can diagnose an early gen3 hg leak is a dual view borescope inspection with a cold engine combined with a pressurized cooling system. There are several videos routinely posted here showing the process. Failure to act quickly to early symptoms can eventual progress to traditional hg problems including a warped head, bent connecting rod or hole in the block.

 

I *don't* know that, and seeing you already so seemingly fixated on that possibility, in a car apparently using no coolant and having started a little rough maybe twice recently, makes me worry that you (not your car, you) are a casualty of the sort of totalizing way it gets talked about here.