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Activist investment firm MHR Fund Management LLC said it increased its stake in Navistar International Corp. to 14.95% as of July 9.

Reuters reported that the fund, founded and run by Mark Rachesky, became the largest shareholder in the U.S. truck and engine maker in June by acquiring a 13.6% stake, edging out Carl Icahn.

Navistar has been struggling for the past year to contain costs of developing a new type of diesel engine for heavy trucks, and has seen its shares lose about half their value in the meantime.

Navistar, facing pressure from investors to sell itself or change its engine strategy, said last week it was developing a new engine which is expected to be ready early next year. On June 20 Navistar adopted a poison pill aimed at keeping outsiders from gaining a stake of 15% or more.

Navistar shares closed at $21.95 on Tuesday (July 10) on the New York Stock Exchange.