Could this be the real reason why the head gasket fail?

As far as I'm concerned they fail for an adequately explained and researched reason: uneven heating and cooling.

That's been popping head gaskets from every engine maker for 100+ years.

 

What's the miles?

How much is slightly? How much (in mm or inch) above top mark on dipstick. Up to 1/4" I'd just let it be.

Long shot...

 

there are many instances where more is not better, and can sometimes cause pain and death

 

Depends on what you mean by slightly.....
I put 4.5 quarts in mine. It always goes right to the bottom of the full line.
When I put 4.75 qts in it, like it calls for, it's just over the top of the line.

Now if you put a full quart over, that could likely be a problem.

 

That’s nothing.

I doubt overfill is causing the head gasket failures. My theory (in case you haven’t heard) is that the carbon-clogging EGR system is the culprit.

 

some think the oil should be changed at 5k intervals, others say the coolant should be changed more often or at least tested.
toyota claims the coolant pump speed control needs updated software, and i think unlike n/a, europe got a tsb.

 

TSB EG-0127T-1014, to be exact.

 

Can you clarify why you think so?

 

The PCV system does a fine job of putting oil in the intake manifold, even without overfill of engine level.

FWIW, I've found putting in the spec'd 4.2 liters after draining the oil (and changing the oil filter), I end up about 1/8" below the top mark on dipstick.

That's after draining with the front of car raised quite high, which likely gets a little more oil out of the sump. With the car level, say on a dealership lift, with the car being level, it might end up very close to the top mark.

I measure the oil with a large, graduated pitcher, pour it in, and just leave it till the day after a drive to check. I've found checking right away, even after running it a few seconds to fill the oil filter, it tends to show low on the dipstick. The few times I've added more at that point, it has turned out to be overfilled.

 

I believe all manufacturers have some factured in a +/- for the amount of oil for the engine.

The engine is no different than any other engine, just because it's in a hybrid car.

 

A pcv sticking open could cause more combustion gas/oil vapor to be drawn into the intake at idle. Normally this will code and the engine will run rough. But not misfire or blown head gasket rough. Luckily hybrids don’t idle much because they just shutoff. Stuck closed the blocked crankcase pressure will blow oil seals, typically the timing chain cover with its block seams poorly sealed with form in place gasket sealant.

The Car Care Nut’s concerns are justified for the above scenarios. Too much oil or oil burning makes stuck open more of an issue.

An oil/gas separator is used on newer engines and automatically drains condensation back to the crankcase. On our engines some go with catch cans in the pcv hose but have to drain them manually. When they are forgotten (most don’t even check their oil level) you get a closed system with bad results or a a big slug of liquids into the intake.

With or without a catch can you still get liquids pooling in the intake. Is it a cause of head gasket fails in a hybrid 1.8L when the 1.8L in a conventional car does not have high incidence of hg fails. Not very likely.