What is the chief cause of head gasket failure on the gen 3

I do both, usually, and mine still runs fine.


iPhone ? Pro

 

Low coolant level means a lot of very compressible air in the expansion tank. This could mean < 15 psi at normal operating temperature. < 15psi means lower than normal coolant boiling point. I think this becomes certainty at some level of low coolant. I have not attempted to figure out how low is too low (yet) but it isn't hard to do. Just need to go find the thermal expansion coefficient of 50/50 glycol/water.

Speculation: Would lower boiling point due to low pressure increase the risk of localized overheating? Sure. Does it really happen? I don't know. If it did happen, would it increase the risk of head gasket problems? This seems possible.

It's certainly true that low coolant is one indicator of a leaking head gasket. I wonder if it is also a cause.

I think the pressurized expansion tank is a dumb design. All cooling systems lose coolant over time. This design might lose a little less than a conventional overflow tank, but it isn't immune. My 2010 had probably never been topped up since new (bought it in Nov 19). The level was most of an inch under the low mark. I bet that's typical.

 

I found both coolant in both reservoirs dropped (from full line) to be slightly below the low lines after maybe 4 years of ownership. I topped them both up to the full lines, and they haven't budged in the subsequent 5+ years.

 

My more advanced cars all have a closed coolant design like this - the Prius, BMW, and modern Porsche. I tend to believe they know what they're doing.
On the extra space in the expansion tank - I believe that will affect how the pressure rises, as you suggest. But as the temperature approaches the boiling point, evaporation and tiny amounts of boiling, if they occur, will turn the water into vapor, filling the volume and increasing pressure ... which increases the boiling point, keeping things in check. So it's not a problem.

 

The system works fine if one maintains the level within specified limits.

Boiling will raise the pressure in a closed system with arbitrarily large headspace when the entire volume of the liquid is brought to the boiling point. In the present case, it won't. The only place in the system where the vapor pressure will be high enough to displace liquid and increase the pressure in the system is at the hot spot. That's not going to be a good thing... that's the exact scenario I am speculating might be related to HG failure.

Just putting this out there in case it draws out corroborating knowledge or experience, that's all.

 

I basically agree.

But say you have a hot spot, and a boiled bubble is created ... while surrounded by cooler (than boiling point) coolant, the vapor in that bubble would almost instantly condense back to liquid, transferring that heat to the surrounding coolant, raising its temperature. So it's not like you'd be getting a pocket of boiled coolant while the rest of the cooling system is cool.

Eventually by this process, yes the whole coolant system would rise in temperature, without having any large scale boiling happening, to the point where a bubble may not be instantly condensed before it makes its way to the expansion tank. The vapor from that would increase the vapor pressure within the expansion tank, increasing the boiling point, so everything would be OK.

Another thing to consider as you're thinking about it is that there is normal variation throughout the cooling system. For example, a typical temperature drop across the radiator of about 10*F is common. Pressure will similarly vary throughout, highest at the water pump outlet.

 

Found this whole thread fascinating! Is there a follow-up thread somewhere?

 

Old and tired coolant can also degrade the rubber layers of the head gasket.

This video shows how this type of gasket degrades when installed with Toyota water pumps.

Basically, Toyota Super Long Life Coolant is not so long lived.

 

How often do you think the coolant should be replaced?

 

If it’s as simple as a wider gasket, hasn’t someone made one of those that could be used as a replacement for the original gasket?

 

As the cideo noted; five years or 50k miles, whichever comes first.

 

I’ve had a 2010 and still have a 2012 Prius, both had head gasket issues. The 2010 had 400,000 and the current 2012 has 320,000. I was loosing antifreeze and hesitation from a stop. I used Bars pelletized radiator heavy duty stop leak. I put 1/2 bottle of pellets without the liquid into lower overflow tank hose. Reattached the hose and in maintenance mode ran engine until no more bubble appeared in the overflow tank. The hesitation stopped, fuel economy increased 5 mpg.

 

sweet

 

My headgasket went out at 140k I didn’t notice anything much other than driving the car a lot and taking it to the canyon and frequent roadtrip. I felt like it was how I drove the car which caused the HG to go out quickly

 

Anything in particular about the way you drove? Given that these engines are so susceptible to head gasket failures, something about use conditions, or driving habits, or maintenance practices must explain some examples failing much earlier than others.

 

Were the EGR and intake manifold cleaned? If so, at what miles?

 

I’ve always been heavy on the gas and kept it mostly on power mode

 

I would bet keeping it on power mode caused acceleration of head gasket deterioration moreso than being heavy on gas…

 

The engine and ECM don't even know the difference between being in power mode and being heavy on gas.

The go pedal is an input to a different ECU, the power management control ECU. The mode buttons are, also. Depending on the mode, that ECU converts how far the pedal is pressed into a number for how much power is wanted.



After accounting for how much power to supply electrically, the power management control ECU is left with a number for how much power to request from the engine. It sends that number to the ECM. The ECM doesn't know how it was arrived at.

 

Drive your car anyway you want too, it's your car.