What's the newest data analysis and results? --- EGR clean prevents HG gasket or is it inevitable

unfortunately, there is no data. data requires controlled testing and record keeping, of which we have none.
what we do have is a lot of experiences reported, good and bad. we do know that not every head gasket blows, if you put some mileage/time on the answer.

toyota says it is badly designed water pump speed software, for which they offer an update (in europe only?)
many guesses here include egr and related assemblies, defective head gaskets, 10k oil changes, infrequent coolant changes or even ph testing, poorly designed pistons and rings, and etc.
how to make sense of all that?

 

A lot of the naysayers give examples, owners that clean their EGR when the odometer's into six digits, and maybe even exhibiting head gasket failure symtoms, such as cold-start shakes, losing coolant. The concept of "interference engines" comes to mind, basically something advantagious is applied to an engine, with the caveat that something else is implemented, to allow that application to work safely.

With 3rd gen Prius, better mpg (a tenth or two of a mpg) was achieved, by running it leaner, and to keep the engine from overheating the EGR system was added. If Toyota had bothered to run a few test cars for 200k miles, we likely wouldn't be where we are.

Same could be said for the low-friction piston rings. Anyway:

^ A lot of times the naysayers will say "I cleaned the EGR, and head gasket still blew". When asked at what miles, it's typically around, well, your miles. Bottom line, just from reports here, my take is it needs every 50k miles, or less. Any more than that, the EGR is compromised, has reduced flow, unbalanced flow and/or overheated flow. Likely all three.

Belated cleaning is still good to do, but definitely don't get complacent. Our third gen YouTube guru @NutzAboutBolts cleaned his EGR and intake manifold around 120k, and is now having head gasket failure, around 180~190.

See the top two links in my signature*, also see this workflow summary (just for the EGR side of it, not the intake manifold).

* on a phone turn it landscape to see signatures

Not sure if it's mentioned in any of the above links, but exclusively for the EGR cooler, I'd recommend a lye solution soak, for fast/thorough cleaning. It's stainless steel, impervious to lye, and really needs something strong, due to the internal radiator labyrinth. For everything else brake cleaner, brushes and rags will do. You definitely DO NOT want to or need to use lye on anything else. It will react with aluminum, non stainless steel.

 

My unsubstatiated hunch: with regular/complete cleaning of the EGR system, at least every 50k miles of the car's life, the head gasket is much less prone to fail.

 

That misstates what it's for, though. The EGR is primarily there to reduce NOx emissions, by reducing peak instantaneous temperatures reached in a very brief part of the stroke. NOx is only formed when the instantaneous peak goes above a certain temperature.

Over the several decades of cars having EGR, the manufacturers did figure out how to get MPG improvements from it also, by tricks like using it to reduce power without closing the throttle as far, so less work is done pumping air across the throttle. That made it no longer a necessary-evil tradeoff of MPG for antipollution, and instead a win for both (a big and important antipollution win, and a more modest MPG win).

It's important to think separately about "instantaneous peak temperatures" and "overheating". Overheating is a macro phenomenon, what temperature the engine materials and coolant reach. The cooling system is what controls that, and it takes in stride bigger changes in heat production when you accelerate, coast, or go from downhill to uphill, than any difference the EGR makes.

If it were not so, you'd be able to use the coolant temperature reading to know when your EGR needed cleaning.

 

I'm under the impression that it's the thermal cycling that is causing the HG issues. EGR blockage will increase crankcase pressure but typically that will result in oil being blown out of seals like the RMS or similar.

You can test for this condition by putting a glove over the oil fill hole and running the engine.

If concerned you could always remove the dipstick and run a heater line tube off it and route it under the vehicle to vent excess crankcase pressure. Just remove prior to your annual inspection .

Also, replacement of coolant in regular intervals should reduce its acidity and its ability to attack the factory HG.

On our vehicle specifically, early replacement of the engine coolant pump (electric) is also needed as I do not believe we have any sort of CEL or light if it fails.

I read a writeup from someone who worked at an independent Toyota shop that stated that 90% of the HG failures in their shop were accompanied by the coolant pump failure.

 

It's likely a combination of things. And the pcv valve takes care of the pressure.....

 

You'd hope. I have no idea where it is on the Prius but in an older vehicle, the PCV would clog and blow the RMS.

 

Underneath the instake manifold, on the front of the engine block.

 

Visual aid:

https://attachments.priuschat.com/attachment-files/2025/06/270395_IMG_6928.jpeg

(courtesy of @NutzAboutBolts, doing the head on his '12)

 

EGR is the one potential culprit that seems to have a direct corellation to the regular-as-clockwork failures at cylinder one: the intake manifold's EGR capillary passage at cylinder is usually the first to fully clog. Behind the scenes Toyota maybe had a similar inkling, they revised the way those passages split off the intake manifold's main EGR gallery.

 

They did that for the snorts-condensed-water-after-cold-soak issue though.