Is that steam? The Toyota Prius faces another snag.

The big number recalls continue at Toyota, more than a year after the maker announced the first safety campaign caused by runaway vehicles.  The latest action involves 650,000 of Toyota’s third-generation Prius hybrid due to a defective coolant pump that can cause the vehicles to overheat and lose power.

Nearly 60% of the Prius hybrids involved in what is officially a service action were sold in the United States, according to Toyota, which says it will cover the cost of any repairs conducted at one of its dealerships.

The maker says the problem involves an electric water pump that can slowly permit air bubbles to enter the coolant lines.  As that happens water circulation slows and temperatures rise, resulting in overheating  of the hybrid-electric componentry.

Initially, that should trigger a warning light, but if the problem is left unattended, the vehicle would drop into a “fail-safe” mode where power would be reduced.  It is not clear from Toyota’s statement whether permanent damage to the vehicle might result if the problem were still ignored.

The problem involves only Prius hybrids built between 2004 and 2007.  The maker subsequently switched to a different pump design.

According to Consumer Reports magazine, the current version of the Prius, launched in 2010, has had some technical problems that have led to an average reliability rating.  Among other things, the latest-generation hybrid has experienced problems with its braking system which resulted in a major recall early this year.  (The defect was first reported by TheDetroitBureau.com on December 24, 2009.)

But while older Prius models ranked significantly higher on the CR reliability charts they have had some problems, notably including widely-reported glitches with their HID headlights.  And a number of owners have complained about braking problems with the Prius model prior to the current fourth-generation hybrid.

Since the latest problem is being labeled a repair action, rather than a recall, it will not count in the growing tally of major safety problems that have struck the Japanese maker, which had long been known for its high-quality products.

Nonetheless, since the first recall for unintended acceleration, Toyota has recalled about 14 million vehicles worldwide, including 11 million in the U.S.

Toyota plans to notify American owners about the overheating problem with Prius in December.

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