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I've had to kind of overhaul my Toyotas engine (1ZZ-FED) as well due to excessive oil consumption.
I failed emissions due to the visible cloud coming out of the exaust.
The bores where still in pretty good condition without scratches and an intact cross-hatch pattern, so I just gave them a little hone, new rings and drilled the pistons.
Also:
A big fault was in the oil controll rings and drain holes.
They are both prone to clog up and seize, causing high oil consumption.
Valve stem seals also often go bad, wich increases oul consumption as well.
This results in carbon deposits everywhere.
Had several mm of carbon and ash on my exaust valves, piston crowns and combustion chamber.
Especialy the stuff on the exaust valves is a ***** to clean off, even with a drillpress and wirebrush.
Not sure if the Prius is the same, but I've heared the valve stem issue and oil controll ring issue are typical Toyota problems...
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