This is why your Prius fails head gaskets

Interesting theory. If there is that much thermal expansion and contraction while driving, it could explain the HG failures.

But the engine is off so briefly and there's so much mass, that I'm skeptical of there being that much contraction in those short times when it's off. There are transducers that will measure that effect. We had them on the presses I worked with for the purpose of detecting jams.

Also, I don't see where the bolt pattern would lead to most of the failures being at cylinder #1, but I do see that that is the cylinder that has the most EGR port clogging.

 

Good question.

FWIW, they're not just dirty; somewhere around 150K they're completely clogged. It's around those miles that engines with unadressed clogging start blowing head gaskets.

I'm a little skeptical of Gasket Master's explanation.

More evidence of EGR clogging being the root cause:

 

it wasn't a problem with gen 2, hopefully gen 4, so, it's not the other side of the coin from good gas mileage, it's just a major engineering mistake on toyotas part.
they aren't what they used to be.

 

Quantitatively, mine at 155k had a flow measurement reduced to around 10 or 11 kPa. Considerably down from the 21 or 22 seen with sparkly clean ones, but a far cry from completely clogged. And that reduction in my case wasn't purely cooler buildup; it was a combination of that and damage to the rotor stop in the EGR valve, which also reduces the flow measurement in an independent way.

Could be my car was the only exception, and everyone else's is completely clogged around 150k. But Occam's razor doesn't allow me to assume that.

 

Maybe carbon coating on the radiator fins within the cooler causes less efficient cooling of exhaust gasses, accelerates (or precipitated) the rotor stop damage? That's the plastic portion with ramp?

 

I know I don't have to speculate on causation, because others will be quick to do that for me, especially if it appears to be weavable into a favored narrative.

Clearly heat softening of the rotor is involved. We can see that Toyota added coolant nipples to the EGR valve itself in Gen 4 and in Prius c, and we could speculate about why they did that. Maybe there are conditions where the valve tended to overheat anyway, buildup or no buildup.

The simple things that can be said without speculation: the rotor stop damage has been observed in multiple valves, it is easy to check for, and it should be checked for, so that the valve can be replaced if the damage is there.

 

If that's so, how is it possible that I'm getting a flow test result of 19kPa at 90% of that mileage? Some of them clog at 150K, some earlier, and some later, depending on circumstances, driving habits, etc.

??? Brief? Not everyone lives in Florida. My engine stops burning fuel (although still rotating slowly) several minutes every time I coast down a longish hill on I-40 in N.C. (just for example; there are plenty of longer and steeper downgrades in the USA), resulting in the coolant temperature dropping tens of degrees---to be immediately followed by working hard to climb the next hill.

 

Yes. That's a good example of the same thermal cycling I mentioned. I-68 in western Maryland has tougher hills than I-40 does in the Tennessee and North Carolina mountains that I more often traverse, I think.