How common is Gen 2 Head Gasket Failure after 300,000 miles?

The coolant should be at 13 PSI, give or take, so way less pressure than combustion, so should be way less likely to cause a gasket failure than cylinder prssure.

 

I'm guessing that a water pump-to-oil (via timing cover) failure path, if it happens, is likely often assumed to be head gasket and never discovered since the timing cover RTV would likely not be examined in great detail, if at all, and would get replaced when the engine is put back together or never seen when the engine is junked...

 

The timing chain cover, as designed is actually "poor design", except for parts cost/parts count and assembly cost, which is what probably matters the most.
A separate piece for the water pump would have ensured that any possible leak is external and easily detectable (instead of mixing with oil and taking out the engine).

I'm not saying that it's what is happening with my car, just saying that it's optimized for cost at the expense of quality and durability, regardless of how common or rare failures there are.

 

Some more data:
Under the UV light, the inverter coolant already in the car also looks Peptobismol light pink, but cleaner than the coolant (which, I'm ashamed to say, was on my "change ASAP" list when I got the car 50k miles ago...).
An unopened bottle of (Prestone, I think) coolant marketed as Toyota spec is also Peptobismol color under the light.
Brand new oil (synthetic) under the light is white, whereas my oil (with only 325 miles on it) is a very light blue or very light purple.

Interestingly, it looks like with the light I have, dye would not be required, at least not for coolants and oil I have, if searching for an external leak.

 

I believe that they all have the same cover design as my car. I purposely put "poor design" in quotations because it all depends on what the design objectives and constraints were.
If the objective was low cost though lower parts count, lower assembly time, etc. then using a single cover is a success.
If the objective was to focus on minimizing potential coolant-to-oil failure paths and some added cost was allowed by the design constraints (which is what I would have picked, but Toyota obviously didn't), then they failed.
I have no idea if that is the failure mode in my car, but it is a possible failure mode and I believe that a failure there could very often be misdiagnosed as a bad head gasket.

 

Yes. I'm planning on pulling a sample through the drain plug and letting it sit in a tall glass.

 

I took a sample from the drain plug without disturbing the oil (without cranking the engine), about 1 quart. The oil has been sitting undisturbed in the oil pan for about 3 days now. I was expecting to maybe get some coolant or water out first, but I did not notice any at all.
The oil is light brow, substantially different, of course, from any oil I have drained from the car before. There was a pretty strong gasoline smell I have never noticed before.
In the container the oil does not respond to the UV lamp at all. when spread thin on the paper it glows the same light blue/light purple under the light as oil obtained from the dipstick.

 

I filled a tall glass with the oil and covered it with Saran wrap. (I'm not very optimistic that it will separate, but time will tell).
Not sure why the oil this time smells so much like gasoline, especially after only 325 miles.
I checked again and no misfire or any other engine codes.

 

It may indeed be VERY reliable, but it's also an unnecessary failure path that could have been engineered even better to completely eliminate a coolant to oil contamination failure mode (at some added cost, every design is a compromise/balancing of design goals. And constraints such as cost.)
If may happen very seldom, BUT because the automatic assumption will often be "head gasket" AND this issue (if it was present) is also always fixed by changing the head gasket, I know that frequency of this failure is undercounted, but I of course don't know by how much.

I would be very surprised if a discussion or much more likely a debate over 1-piece vs 2-piece cover didn't take place during the design phase.

Maybe this could help: A leaky fuel injector could result in very similar "morning knock" symptoms as a leaky head gasket and be misdiagnosed as a head gasket. But in this case after the (unnecessary) head gasket replacement the symptoms didn't go away and (hopefully) the proper cause would be found (although at least some DYIs would probably just assume that they didn't do that head gasket correctly).
On the other hand, a misdiagnosed timing cover to oil pan coolant leak would likely almost never be discovered because 1) after timing cover removal the evidence is probably not examined and is quite possibly destroyed during timing cover removal and 2) fixing the head gasket also fixes a timing cover leak.

 

The timing chain cover has a coolant passage that is separated from the crank case with RTV. RTV can fail. Is it likely on a Toyota built engine? Probably not likely, but all it takes is one careless worker to hurt the odds on those engines. Either way the chance is not zero and any data will underrepresent the failures for reasons I have already explained.
On an engine that has had the timing chain or other components replaced or an oil leak fixed by DYI or "shade-tree" mechanic, whether mobile, independent shop, or dealer), the odds are likely much higher (but could still be very low). Any "data" will again underrepresent the failures.