LONDON -- A broken valve prevented Juan Pablo Montoya from challenging Ferrari's Michael Schumacher for victory in Canada last weekend, BMW motorsport director Mario Theissen has said.

Montoya, who wrapped up testing in Spain this week for the BMW powered Williams team, was running second in Montreal when his engine blew.

"On Wednesday we dismantled the engines," Theissen said. "Juan Pablo's engine stopped in the race due to a broken valve. On Ralf's car, a problem with a piston appeared after he had crossed the finish line.

"It actually was not a failure of the piston itself but of the operating conditions."

It was the second race in a row that Montoya, who started on pole in both Monaco and Canada, has been sidelined by engine problems while chasing a podium finish.

The next race is at Germany's Nuerburgring next week and BMW will want to avoid another failure at a circuit considered home for rivals Mercedes, McLaren's partners.