Carl Icahn increased his stake in Navistar International Corp to 13.19% as of July 11, up from his previous stake of 11.87%.

Reuters reported the move comes a day after MHR Fund Management LLC — largest shareholder in the U.S. truck and engine maker — raised its stake to 14.95%. On June 20, Navistar adopted a poison pill aimed at keeping outsiders from gaining a stake of 15% or more.

Icahn, a billionaire investor known for shaking up companies and advocating sales, had pushed for a merger between Navistar and rival Oshkosh Truck Corp. early this year.

Navistar, parent to Monaco RV LLC, 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.

The company, 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.