How to prevent the 3rd Gen Prius Headgaskets failure!

Especially, I would suspect, rapid thermal cycling, such as asking the engine to work hard soon after a fully cold start. I let it warm up for a minutes or so before attacking my immediate hill, then try to keep engine speed down

It's impractical to avoid significant thermal cycling if you live and travel in hilly areas a lot. Coast down one hill with engine off for a couple of minutes, maybe have to stop for a stoplight or stop sign at the bottom, then launch up the next hill hard enough not to get honked at by the 18-wheeler behind ....

 

It seems to me that thermal cycling would have to be something Toyota thought about, though. In my Gen 3, once the engine has started and run to operating temp, the car seems not to want to let it cool much below that. While there are surely emissions benefits from that, it also keeps the thermal cycling in a narrower range.

The car's strategy is a little interesting, in that if it figures out you are parked and using cabin heat, it can gradually drift the engine-start and -stop points downward toward a temperature just adequate for heating. The details were over in the Gen 3 hunkering thread.

I offered, there, a fiddly method for overriding the car's strategy to achieve a super-extended hunkering time if you're not using the car's cabin heat. But doing that would mean much wider thermal cycles than the car's strategy would normally allow, so it would be a use-at-your-own-risk deal. Certainly, if I were stuck on a blizzard-closed roadway, and an estimate of time to rescue and fuel in tank looked like the normal strategy wouldn't last long enough, I'd consider it.

 

I posted a video here a couple of years ago to follow up on the thermal cycling issue. I used a video screen capture of real time data with short voice overs for driving modes and loads.

I started by driving with ac to get a fully warmed engine at 190f. Then a Parked in Ready like a car wash or waiting at the mall for ten minutes which dropped it to 150f. Driving less than six minutes raised it to 200f.

So it can thermal cycle by as much as 50F (eg 150-200F at 5:55) especially after a long "idle" (EV). I often see 20 degrees delta t in stop and go driving with a healthy hv battery.

Time Temp ReadyMode

-10:00 190F Park in Ready (prewarmed by driving to 190f)
00:00 150F Park in Ready (eg waiting, car wash, taxi, delivery)

00:10 155F Drive
01:50 170F Drive
04:00 180F Drive (uphill)
05:55 200F Drive (top of hill)

07:05 185F Park in Ready (home)
17:00 158F Park in Ready

 

The comparison to the behavior documented over here could reveal new interesting stuff.

Those results were all in Park (it was a hunkering thread, after all), and in cold conditions where heat would be used.

You don't mention the conditions for your observations, but you mention "with ac", so I assume warmer.

I've added a Celsius column to your numbers since it's obviously what the Toyota engineers design with (all their start/stop and fan on/off setpoints, etc., are nice round numbers when you read them in Celsius).

-10:00 88C 190F Park in Ready (prewarmed by driving to 190f)
00:00 66C 150F Park in Ready (eg waiting, car wash, taxi, delivery)

00:10 68C 155F Drive
01:50 77C 170F Drive
04:00 82C 180F Drive (uphill)
05:55 93C 200F Drive (top of hill)

07:05 85C 185F Park in Ready (home)
17:00 70C 158F Park in Ready

Your numbers at 0:00 and 17:00 are interesting, because my 2010 liftback would never allow the temperature to drop below 80C without starting the engine to warm it back to 90C (in Park), where it would then stop the engine, and cycle between those two temperatures for as long as I cared to watch. That was with the heat turned off.

With the heat turned on, in the same conditions, the car would start out maintaining 80 to 90 C the same way, then gradually drift both setpoints downward, so after sitting long enough in Park, it would be cycling the engine on at 50 C and off at 60 C. But then if I turned the heat off, the next engine cycle would not stop until 90 C, and it would again maintain strictly 80 to 90 C.

So it could be interesting to know what your HVAC settings were, and the outside conditions. There might be some more control rules in there that winter hunker conditions didn't catch.

The range of thermal swing upward from the nominal "operating" temperature, depending on the driving load, is pretty much a common feature among cars. The Prius considers itself warmed up at 80 C, stat begins to open 80 to 84 C, is fully opened at 95 C, fans start at 95 C, so your 93 C at the top of a hill isn't that unusual, even pushing 100 C wouldn't be too weird on a longer hill, higher load, or hotter day.

So there's an expected thermal cycling range of around 20 Celsius degrees (36 Fahrenheit ones) that isn't much different for a Prius than for any other car. It seems like where we should be looking for the Prius-specific difference is in how much wider that range gets because of the engine being allowed to cool. Which, as we see, depends at least on HVAC settings, and maybe on some other things.

Under the conditions where the car maintains strictly 80 to 90 C, it's pretty much matching the range any ordinary car would see in typical driving.

For evaluating any effect of this on head gaskets, I honestly don't know what temperature should be called "zero". If you look at swings between 66 and 93 C, you say yikes, 30%, but if you're thinking in Fahrenheit, it only looks like 25%, and if you look on an absolute scale, 339 to 366 kelvins only looks like 7%. I definitely don't know what tolerance the engineers think they need to hit, or what calculations they have to use to get there.

 

what does toyota see as the root cause, the pistons and rings?

 

Have they said anything about head gaskets?

they’re on record acknowledging egr clogging (albeit saying “the vast majority” will not experience this) and the low tension ring failure (but only up to 60k miles).

So, why are the head gasket failures almost always between cylinder one and two? There’s an easy answer.

 

that's what i would like to know. i'm not sure if rj is inferring by toyotas actions and responses, or has made an official statement.

 

All interesting. In my experience, coasting down a long hill in winter, the temperature easily drops from about 88°C typical at the top down to 70°C or so, and the engine still doesn't automatically restart. Dropping below 75°C is frequent on downhills.

I've never seen above about 95°C in any circumstance. (That occurred trying to follow a friend's non-hybrid up a hill in North Jersey in early spring, oddly enough.)

 

The fans make a noticeable difference. Those click on at 95 ℃ and then usually the temperature reading turns right around and decreases, unless under the hardest conditions.

 

Did I missed something?
What would be the easy answer?
I'm not joking, could you explain a little, please?

Mat

 

There is no easy answer after the engine has miles on it.

 

Sorry, being obtuse. There are small diameter EGR passages in the intake manifold, one at each port. They clog progressively, and the clogging is most advanced at cylinder one, then two and so on. Perhaps due to the way the EGR gasses are introduced to the intake manifold, down at the other end, the cylinder four end.

So, cylinder one is first to be starved of EGR gas, and the head gasket failures are regularly at the wall between cylinder one and two.

 

I disagree! You are not like that

I knew, they clog unevenly, but it was not obvious to me, that it mostly happens at the first and second cylinder - the most distant ones from the EGR inlet on the intake manifold.

I can imagine, that the uneven EGR supply could have an impact on the combustion in the cylinders (the Engine ECU knows nothing about this unevenness), but how could this lead to a head gasket leak between the 1 and 2?

No idea...

Mat

 

I think Mendel wanted to leave it as a rhetorical question so you'd make the explanatory jump yourself.

By Mendel's tally, there's a preponderance of manifolds more clogged at the cyl #1 end, and a preponderance of gaskets failed at that end. Both tallies are hard to check; PriusChat is many things, but a statistically sound databank of manifold clogs and gasket failure modes among 3 or 4 million gen 3 Prii sold globally, it is not.

But if we say for purposes of argument that both those tallies are accurate and representative, then it wouldn't be insane for a person to wonder about a possible connection, and start thinking of some experimental way to get beyond simple correlation and demonstrate causation.

 

That sounds like a Toyota engineering responsibility.

 

There's no question Toyota hires lots of people who know how to do that stuff and pays them to do a lot of it.

But sometimes the person pushing a particular claim is somebody outside Toyota, so that's where the responsibility lies in that case.

 

Maybe due to my lacking knowledge and experience, I am not able to make the jump myself

I hoped, someone did it before and I was not aware about it.

If both this "tallies" are true, there would have to be an imaginable mechanism, that the one would lead to the other.

As I said - I have not enough knowledge or experience to imagine something like this...

Maybe the 1 and 2 are getting hotter as the 3 and 4? And are experiencing more temperature changes and are causing more tension on the gasket?

Mat

 

There's definitely an imaginable mechanism. All that's still missing is the legwork to pin down what the real rate of HG failures in Gen 3 cars is, and what proportion of those can be shown to have a single cause, and whether that cause can be shown to be the one suspected.

EGR in a cylinder prolongs combustion, so the temperature and pressure have a lower instantaneous peak but a longer duration, extracting the same energy from the fuel overall. It allows the ECM to push things like spark advance advance further before hitting the limit where the fuel charge might detonate. Detonations are mechanically stressful on the engine.

The car self-monitors for EGR flow, and if it detects a problem, the ECM dials back the spark advance to a fail-safe level. But uneven clogging of those manifold passages can avoid detection by the flow monitor, as there is only one sensor, for overall flow. If the system can see an acceptable overall flow, by flowing too little EGR into some cylinders and too much into others, the ECM won't see a problem, and won't go to fail-safe operation. So it could be that the cylinders with deficient flow will be at detonation risk.

 

Detonation is the same as knocking - right?
Should the knocking sensor not detect that?
Mat

 

https://dynojet.zendesk.com/hc/en-us/articles/360004003513-What-is-Knock-Detonation-and-Pre-ignition-

Knock is a recognizable sound that can reveal either detonation or pre-ignition. There is a knock sensor on the front of the engine block, which is able to trigger the ECM into protective actions when measurable knock has been detected.

The ECM is also programmed to manage EGR, and to monitor for EGR problems, in order to avoid creating prolonged conditions favoring detonation. It seems the engineers have not hung all responsibility on the knock sensor.