Need help with P0304

I am in the same boat with a 2013 Persona I didn't do a head gasket change I changed the whole motor and was back to the same noise in less than 10,000 mi basically I gave up on the car I have two generation 3s like yours with the same problem and four generation twos with no problems It is obvious to me to drive the generation twos and leave the generation 3s alone I do not want to spend the money to tear apart the engine that's in the generation 3 that was just installed like 12,000 mi ago It is very disheartening but it was expected just to prove a point this was done I am very sorry you are going through this but it seems this is very common in just this engine series apparently we'll see how the two liter AL does but I'm not banking on that being much better I don't really know what to tell you they'll be lots of people coming along to help you spend your money and try to tackle the problem It may not be tackable at this time or you may not want to spend the money The big mistake here was buying the car with 250K on it or whatever you said it was That's a big red flag for this model so it seems at my point in life I'm not interested in running down red flags and problems I've been with Toyota for 47 years so there's always that 2010 and Forwsrd is planned obsolescence and Toyota and a few other manufacturers have implemented this very strategically and very intelligently The people who they are screwing over now are generally people that know no better so it's a win-win.

 

That kit tests for carbon monoxide in coolant? That is not the usual head gasket failure mode; it’s more typically coolant leaking into cylinders. How’s coolant level in reservoir; monitor it for a while? Also consider boroscope inspect combustion chambers, looking for coolant evidence.

that sounds like one well used engine btw. How’s oil consumption?

 

At some point you may run into a camera that you can rent or borrow that looks backwards after you stick it in the hole because you want to look up at the gasket mating services of the cylinder head and block assembly yeah I know it's kind of crazy but they make cameras that do that And of course I'm imagining your EGR circuit is all cleaned and up to par.

 

A borescope inspection is the definitive test of early stage head gasket failure since there will not usually be massive coolant loss. This has you pressurizing the coolant tank to about 15 psi and watching for leaks inside the cylinder.

A rental kit from any auto parts store will let you create pressure, but none of their adapters currently fit the Prius coolant reservoir. An adapter that is known to work is the CTA Tools 7103 which you may need to purchase.

You'll also want to get a full HD dual lens scope. The side camera will let you perform the critical check for coolant droplets seeping through any gaps between the head gasket and the head/block. These days, a $20 unit can be purchased that employs a user-supplied smartphone as the display.

 

Unrelated?

 

I first got it done at about 170K by a "professional", and the repair failed less than 1K miles after that.

 

If the block or head was warped and a new head gasket was installed - that failure would be predictable.

@Haisam, there needs to be more testing/troubleshooting and less 'shot-gunning'. Every-time a problem is shot-gunned, your introducing more variables into the problem. At this point, you don't know if one of the things you did isn't contributing to the overall problem your having. While I do understand that your resources are limited, wasting time and money on parts and labor that doesn't resolve the matter may be adding to your issues.
At this point, I'd borrow a compression tester from the local parts store and do a simple compression test. That'll tell you if you have the compression required for combustion and go from there.
BTW; I would've never gone after the EGR for a P0304 code because an EGR issue would effect all firing cylinders - NOT just one...

 

The head and block were both intact in this particular instance; the reason that first repair failed was a faulty installation. Findings detailed at the embedded link in #10.