The woes of electric-assist power steering keep mounting. Cases in point: The 2007-2009 Mazda3 and 2003-2005 BMW Z4. The NHTSA today announced an investigation into both cars, with a combined 140 incidents and four crashes between them due to a loss of power steering. Earlier this week we broke the news of a potential issue with the electronic side of the 2011 Hyundai Sonata's electric power steering system, and Lexus recalled the LS in May for yet another type of electric power steering failure.

While the BMW Z4 was one of the first cars to introduce true electric power steering assist (EPAS) in 2002 (though it was beaten to market by 12 years by the Honda/Acura NSX), the Mazda3 uses an electro-hydraulic power steering system (EHPAS), which is a sort of hybrid between traditional hydraulic power steering and the pure electric-assist systems.

The problem, according reports filed with the NHTSA, is a loss of power steering boost at speed. A total of 48,764 BMW Z4s and 293,787 Mazda3s are subject to the investigation, which has led to the crash of one Z4 and three Mazda3s, with another 33 incidents for the BMW and 107 for the Mazda.

Mazda has actually had a technical service bulletin (TSB -- #06-002/09) out on the Mazda3's propensity for EHPAS pump failure since 2009, which notes that the problem is not relevant to Mazdaspeed3s, and only particular ranges of Mazda3 VINs. It also applies to the 2007-2009 Mazda5, however, so don't be surprised if it gets rolled into the NHTSA investigation as well.

The problem, according to Mazda, relates to contamination of the EHPAS system due to corrosion inside the power steering line, which sticks in the pump and slows its operation, reducing or cutting all assist from the electric pump, which puts the system into a "fail safe" mode.

The cause of the failure with the BMW models isn't as clear, though it occurs at speeds of 45 mph or more and at ambient temperatures above 75 degrees, according to the NHTSA.

We'll keep you posted on the investigation if it leads to a full-blown recalled of the affected vehicles.

[NHTSA, Mazda]