Lexus USA has filed its official papers (PDF Link) for the LS/IS/GS valve spring recall, here is the full list of affected vehicles:
Also included in the recall document is a timeline of how the valve spring fault was discovered, which I’m reprinting here in full after the jump.
March 2007 — September 2007
Toyota received a field technical report from the Japan market which indicated an abnormal noise and a large amount of engine shaking when the engine was started. Toyota investigated the broken returned valve spring from this vehicle. As a result, it was found that there was foreign material (Zirconia) at the starting point of breakage, and the fracture surface appeared to be the result of fatigue. Toyota investigated the production process of the spring material.
It was presumed that Zirconia applied to the surface of the casting furnace wall could be peeling off and mixing with the spring material. To eliminate this possibility, the casting furnace was improved in September 2007.
October 2007—December 2007
In examining another broken returned valve spring, Toyota confirmed silicon oxide at the starting point of breakage. Silicon oxide is a by-product of a secondary refining process. Although silicon oxide is eliminated during the production process of the spring material, it is difficult to completely control its residual volume.
Because the effect of residual silicon oxide on the strength of the spring was not clear, the inspection method to detect it in the material was enhanced in December 2007. In addition, Toyota started investigating the effect of silicon oxide on the strength of the spring.
January 2008-August 2008
The likelihood that foreign material of a size that would affect valve spring strength would be introduced in the manufacturing process was considered to be small, and no trend was believed to exist.
However, considering that it may not be possible to completely eliminate foreign material from the production process or detect all micro-sized objects during the inspection process, Toyota engineered and then changed the wire diameter of the valve spring from 3.3mm to 3.4mm in August 2008. At this time, Toyota continued to monitor field technical reports.
The number of reports of broken valve springs was relatively small, and the vast majority reported abnormal engine noise, rough engine performance, and no start conditions readily noticeable to vehicle operators.
September 2008—September 2009
Toyota continued to monitor field technical reports, and also continued the investigation the effect of silicon oxide on the strength of the spring. To further enhance valve spring strength against the possibility of foreign material inclusions, Toyota changed the wire diameter of the spring from 3.4mm to 3.5mm in September 2009.
October 2009—June 2010
Toyota continued to monitor field technical reports.
It also continued to investigate the effect of different size micro-inclusions in the material of the valve spring on the strength of the spring. Even though there were no reports of any crashes or injuries, the number of reports of broken 3.3mm diameter valve springs produced prior to August 2008 increased and the number of reports of total engine failure also increased.
As a result of this investigation, it was found that there is a possibility that the strength of valve springs with a wire diameter of 3.3mm may be degraded by a micro-foreign object in the material of the valve spring, causing the spring to break.
It was also confirmed that valve springs with the wire diameter of more than 3.4mm are of sufficient strength to protect against micro-foreign material of the size that may be introduced in the production process.
Toyota decided to conduct a voluntary safety recall of all vehicles with the subject valve 3.3mm diameter valve spring within the affected range.
This safety recall will also be conducted in Japan, Canada, Australia, Europe and other countries.