Hi Frankie
The oil consumption issues affecting 1zz 1.8, 3zz 1.6 & 4zz 1.4 engines is caused by a combination of several issues, leading Toyota to make several changes to the engines up to around mid 2005 where they managed to cure the issues,
1. Insufficient oil capacity, pre 2005 engines only came with 3.7 litres of oil, this caused the oil to remain very hot all the time reducing the cooling affect the oil has on the back of the pistons. The continued high oil temps causes the oil to degrade and sludge excessively between service oil changes.
Toyota increased the oil capacity from 3.7 to 4.2 litres reducing the oil temps in the engine.
2. Poor piston design, the rear of the piston only had 4 oil ways allowing the hot oil coming from the piston ring oil scrapers to pass away from the piston rings. This meant that the oil stayed at the hottest part of the piston crown for too long exacerbating issue # 1 which in turn allowed the oil ways to sludge up. Once sludging started the oil would eventually start to burn and harden around the rings this started to wear away at the bores eventually misshaping them. Due to the Nikosil coating applied to the bores they should not be rebored or honed meaning Toyota replaced any block showing even the slightest of damage.
Toyota redesigned the pistons to include 8 oil ways which allowed the oil to pass away quicker.
3. The use of poor quality or overly thick oils (thicker than 5w30) increased the likely hood of the issues above occurring. Toyota noted that even dealers where using oils such as 10w40 and quickly pointed out that although the owners manual listed 10w40 it was for warmer climates and that 5w30 was the preferred grade and insisted that dealers use 5w30 when servicing and repairing #zz engines.
4. Poor piston ring design, In an attempt to reduce friction, increase efficiency and lower emissions the engine designers profiled the piston rings to a sharp point reducing the contact point on the bore, this on its own only increased oil consumption slightly but when combined with issues 1,2 & 3 caused the piston rings not scrape the overly thick oils effectively.
New piston rings where designed with a larger contact area and higher ring tension.
Initially Toyota fitted new blocks along with modified pistons, rings, thermostat ( to lower engine running temp ) and a modified dipstick ( to show the new 4.7litre levels) they also recovered the old blocks eventually leading to them switching the approved repair method to remanufactured short blocks fully assembled with the modified parts.
Cheers
Devon
