Rattling my cage

General! We have fired the parts cannon and are out of ammunition! But the enemy is still advancing and seems to be in full force!

Simply block the egr flow to eliminate egr. Egr rarely causes misfires anyway and causes rough idle only if stuck open.

95% head gasket. Even without obvious coolant loss, with negative combustion gas in coolant and or with negative leak down test.

Dual view borescope is the weapon of choice.

 

If and when you do the head gasket, do a full EGR clean as well. See first two links in my signature (on a phone turn it landscape to see signatures).

 

were the 2015 models prone to HG failures? I thought that was solved by 2015 model year?

 

the theory goes that they got new piston and ring design, so less egr build up. but it still needs to be cleaned.
or if you believe toyota, it's a coolant pump software issue.
or if you believe some posters here, it's the 10k oil changes.
or if you believe the car care NUT, it's the acidic coolant and toyotas lack of coolant change recommendation frequency

 

Gasketmasters latest videos recommend preemptive hg changes on all gen3s and rebuilt once they blow (“in stock”).

Rebuilt gets you another 150k miles with a better hg (even from Toyota), better pistons, rings and probably the better intake and egr valve. There is no doubt the gen3 low tension rings cause oil burning or excessive oil burning which multiplies carbon buildup and preignition.

Many 2007-15 Toyota models including Camry, Corolla, RAV4, Solera, Matrix and Scion also were oil burners with low tension rings but only Prius and a low volume Camry hybrid had a thermal cycling Atkinson cycle engine.

 

Just editorial:

Gen 4 fixed the EGR system "shortcomings", but had the misfortune of Exhaust Heat Recover system failures, leaking coolant, this time just into the exhaust, but with protracted neglect could also lead to head gasket failure. More than a few were also misdiagnosed, mechanics seeing (EHR) coolant loss and changing head gasket.

Gen 4 press release, with "revised" EGR system mention, attached:

 

The press release does mention "new EGR system with cooler" as a bullet point. The gen 4 adopted a "catalyzed EGR" system, reflecting industrywide research that was being conducted and published during the gen 3 development years.

 

Translation: they connected the EGR system past the catalytic converter, instead of before it. Presumably with the aim of having less carbon in the system. They also more or less doubled the cross-sectional area of the cooler.

Just running a few of the prototype gen 3's, say for 200K miles, and they would have seen the excess carbon build-up in EGR system. And the runaway oil consumption due to "low tension" rings. And likely blown head gaskets.

 

The "translation" may not be necessary; the published engineering literature, which came from the research being conducted industrywide, kind of covers it.

Moving to catalyzed EGR changes the chemical constituents of the exhaust gas and changes its effects on combustion; that was a significant area of the research.

The increased cross-sectional area of the cooler and the rest of the plumbing follows directly from the lower pressure of the exhaust gas past the cat, and what cross-sectional area is needed to obtain the intended flow at that lower pressure.

I have not said anything here that Mendel hasn't seen before.

 

Still, 3rd gen EGR system will be near-completely clogged by 200K miles. 4th gen system with over 300K, just a light dusting*. That's engineering advance for sure.

* reported by @jerrymildred, from work experience at Tampa Hybrid.