The explanation in this video is the thing that makes the most sense to me. It's sort of the other side of the coin of the good gas mileage. Put those saved pennies into a new head gasket piggybank. All the EGR cleaning in the world is not going to save us.
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.
So in the video he mentions that the Corolla and other cars share almost exactly the same engine as the Prius theirs being the 2ZR-FE. It appears the Corolla has had an EGR system since the early 2000s. However despite millions of Corollas out there I don't think they go through head gaskets nearly as often as the Prius. These guys (Gasket Masters) whole business seems to be replacing head gaskets on the Prius. I think Toyota is probably aware of the root cause of the head gasket failures being the numerous thermal expansion cycles and the engineering challenge that poses. If it was due to dirty EGRs why wouldn't they just specify an EGR cleaning as part of 100k service or something?
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.
I've heard people say their head gasket blew at only 100k miles. I just pulled my parents' Prius' EGR pipe at 90k miles. It was dirty but nowhere near being clogged. Btw I threw it in the ultrasonic cleaner for 10 mins with some Simple Green Aircraft and it came out clean as a whistle, could see the reflection around the bend. But I can't really see the EGR being fully clogged at 100k miles based on what I saw. Art's automotive has a page on head gasket repair. The diagram of the engine block shows that the temperature gauage is on the opposite side from cylinder 1. Not sure if that could have an impact, but it would mean any overheating in cylinder 1 would be less likely to be detected. He also doesn't know why it happens: "However, I’m convinced that not all Priuses with blown head gaskets had an overheating incident. I’ve seen many with no codes in history, no burnt smell in the oil, and nearly full coolant. Why did these fail? I figure that it’s either design or assembly. I’m leaning toward assembly since many Gen3s go 300K miles and never have a problem.". If it was a clear cut case of EGR clogged > head gasket failure I don't think a shop that works on as many Prii as Arts would fail to make the connection. Prius Head Gasket Repair
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.
A lot of the miles on mine were done on I-70 in the winter. Basically like 10-20 minutes of WOT up over a pass and then coasting down the other side, maybe some B mode, over and over. Have to imagine that was a lot of thermal cycling Here's a page where MG and Lotus owners speculate about the causes of their head gasket failures. One of the explanations is the 'Thermal Shock theory": "What is the cause of "so called HGF" in the K series? Well it is a complicated issue, there are many contributing factors, but the main cause is the distortion the aluminium engine suffers under the thermal gradients that occur particularly during warm up and warm down cycles." Why do head gasket failures occur?
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.
how so? I mean, in theory there could be a retrofit kit made for those who rebuild or buy a new motor
4th gen did such a revision, and the head gasket failures went away. Unless coolant leaks in the exhaust heat exchange are neglected long enough.
Hence, the Gen 4 with the EGR gasses burned off post CAT. No major redesign needed just a couple engineers on their lunch break with a good idea.
it's possible on the latter engines, yes but in my 2017 I've seen the coolant so low the dealer added more and it wasn't even close to 100k.
