clog inside the intake manifold....

I never had the rattle on cold starts until the headgasket failed.

 

Better clean before engine blows

 

If a clogged EGR was the sole reason for head gasket failures, I would have expected to see one of the following conditions with my failed headgasket:

1) deterioration by cyl #4, which is the closest to the EGR circuit
2) equal deterioration of the entire headgasket.

Instead, it was the area between cyl 1 and 2 that failed, and the head bolt nearest the failed location had lost torque.

 

I can’t speak to your particular vehicle, but if the dealer has diagnosed the manifold to be a problem, you need to correct that issue before moving onto other ones. In certain states, the manifold is covered for 150K.

On my vehicle, I only experienced the knock (misfire) at startup until I experienced a headgasket failure.

 

Watch the @NutzAboutBolts videos, pinned at top of 3rd gen maintenance forum. There's intake manifold cleaning, EGR pipe cleaning (a good one to start on), full EGR cleaning, and Oil Catch Can install is good too.

 

Cylinder #4 is closer to EGR, yes. From what I remember on the forums, when EGR is partially plugged, cylinder 4 usually gets more EGR and usually cooler than the rest of the cylinders. The Cyl 1 channel is normally the most restricted and has the most carbon buildup.

With the IDs of the EGR channels the same, even without deposits, a longer channel would have more restriction than a shorter one.

 

Sure, I can imagine that possibility. But by that logic, the gasket should reflect this.

 

I'm not sure I understand. Which channels were the most restricted?

Pixel XL ?

 

My EGR cooler was not clogged. I had cleaned it 5K ago, and even then, it was not clogged.

I have not personally examined a headgasket that was removed from a car with a "clogged" cooler. But every failed Prius 1.8L headgasket picture that I've seen, has shown deterioration between cylinders 1 and 2. So, for that reason, I am skeptical that the EGR is the true cause of the headgasket failures.

 

You may be right..

Not all head gasket failures come from EGR restrictions obviously. Even cars without EGR blow headgaskets. I do think a clogged EGR accelerates head gasket wear due to the resulting greater combustion temperatures and pressures. A failing water pump is another contributor.

Pixel XL ?

 

I have a couple theories here that I may want to add here.

Little things do add up. Head gasket failure can be caused by one bad system, like 1) increased combustion temp with the lack of EGR gas, 2) increased combustion ratio with regular octane gas-- from induction of fluid/oil or carbon buildup causing pinging, 3) over heating with failing water pump or coolant leak etc. Now what about but a little bit of each? All of the above are happening in our little 1.8L engine with enough age and mileage on it. So one may not have a fully clogged EGR cooler or totally dead water pump, but over time and mileage, things do happen, just usually doesnt happen to Toyota engines, well, until recently.

A second theory to explain why is the head gasket always fails between cyl 1 and 2, is because the cooler coolant from the radiator lower hose goes back into the engine from cylinder #4 and flows out of the front of engine at cyl #1. 3 and 4 gets cooler coolant while 2 and 1 gets hotter coolant.

I am trying to go by some of the readings here in Priuschat and old memories from school so I can be wrong LOL

 

I like your theory RMB. The missing data, in my opinion, might be found on a Cylinder Head Temperature gauge similar to what one would find in piston powered aircraft.

 

Isn't it possible that bolt "lost torque" (tension, actually) only because the gasket disintegrated out from under it?

Thanks for your expert write-up of your expert work repairing this scary problem!

 

I suppose that is a possibility, too. There are multiple possible reasons for the failure. But the moral of the story is - it is a very real possibility on the 10-15 Prius at high mileage, so buyer beware.

 

Is it possible to check head bolt torque without removing the head?

Pixel XL ?

 

The issue is that the head bolts require angle torque. 36 ft-lbs + 90-deg + 45-deg. There is no way of accurately re-torquing the bolts without removing them and starting from scratch. In theory, MLS headgaskets should not require any re-torquing.