Number 1 cylinder misfire

How many miles?
Engine coolant level stable?
Ever cleaned the Exhaust Gas Recirculation circuit, and intake manifold?

Just a heads up: this is very often head gasket failure, coolant getting into cylinder, and carbon-clogged EGR is a possible factor in its demise.

 

Inspected EGR? By 200k it's pretty much guaranteed to be clogged.

Maybe try a leak down test.

The head gasket failure are almost always cylinder 1 and/or 2.

EGR gasses, which help to cool the combustion chambers, enter from cylinder 4 end of the intake manifold.

 

A miracle.

 

How many miles were on those old plugs, and where did you get them? Any chance they were fake?

These Denso Iridium SC20HR11 are "double" Iridium plugs, so there is a little Iridium spot on the anode, which should show virtually no wear, maybe even up to 200k miles. But the fake ones aren't actually made with Iridium, and will wear through in a few 10's of k's of miles.

 

What did you exactly inspect?

 

So I'm going to assume someone has cleaned out that stuff you've inspected. It would be a miracle (as Mendel mentioned) to have that not clogged by 200k miles

As with the spark plugs, most people only use the Denso or NGK brand ones, as they are the most compatible with the engine.

I'm a big believer in the head gasket failure after 200k miles. This unfortunately is just the nature of these cars. But having a misfire could be multiple things. You'll just have to slowly go through all the diagnostics associated with spark and fuel delivery

 

Mine also is relatively clean at 154,000, both according to flow testing in Techstream and confirmed with a recent visual check (though I didn't visual the cooler).

One things we don't do a lot of at PriusChat is disassemble randomly selected non-problematic cars to find out the true population incidence of our favorite maladies. Miracles might be more common than we think.

 

Still yielding lower to mid 60 mpg on mine so gas quality havent been affected at this time.

Empty OCC every week? There’s definitely an underlying issue happening within the system.

 

Ditto

 

I did my 2010 back when the car was around 100k miles. It was problem free at the time and I just wanted to do some routine maintenance. Mine was not a miracle, it definitely needed cleaning.

 

Yeah, there was recently a Gen 3 about the same age and miles as mine on here that was severely occluded (and a flow test showed it).

So there are significant differences between cars, and what accounts for them hasn't been figured out.

 

Others may find it helpful if you listed the gas brands, and the related things that manifested by using each (milky stuff, excess blow-by, etc)

Curious about the brand and weight of oil you're using as well.

 

The OP does realize there is an EGR cooler behind the EGR valve, and that the cooler clogs up, right? Carbon build-up in the EGR tube and on the valve looked unimpressive to me, but my cooler was almost completely clogged at 177k miles (2010 Prius).

Prius Gen 3 Major Maintenance: EGR, manifold, spark plugs, Oil Catch Can | Page 5 | PriusChat