Wells Fargo Securities analyst Andrew Casey said: “The departure of Ustian is not a surprise after the failure of the company’s engine strategy. The surprise is the length of time that it took for this to happen.”

For much of the past year, Navistar has been struggling to win approval from the U.S. Environmental Protection Agency for a new diesel engine technology that would lower emissions of nitrogen oxide, a pollutant linked to asthma, without using urea. Urea is a raw material used in the manufacture of many chemical compounds, including ones used in automobile systems.

Navistar won a reprieve in early August after U.S. regulators submitted a final rule allowing it to continue to sell engines that do not meet U.S. emissions standards as long as it pays fines.

Last month, Ustian reversed course and said the company would begin using urea, a move that amounted to adopting the dominant technology and one used by rivals including Paccar and Volvo. To do that, Navistar will begin selling trucks with engines made by Cummins early next year.

Navistar has also become a target of activist investors, with about 45% of its outstanding stock held by three big investors: billionaire Carl Icahn, hedge fund MHR Fund Management and asset manager Franklin Resources.

The company adopted a “poison pill” defense in June, intended to keep any investor from acquiring a 15 percent or greater stake in the company.

Navistar’s special committee of the board has also hired Goldman Sachs Group for strategic advice, and used executive search firm Heidrick & Struggles to identify interim CEO Campbell, people familiar with the matter said.

Campbell, 66, was Textron’s CEO from 1998 to 2009. He previously spent 24 years at General Motors in various executive roles.

“Lewis Campbell is a high-caliber executive who brings to Navistar deep and broad strategic, technical and operational skills and a proven track record of leadership with global industrial companies,” said Michael Hammes, Navistar’s independent lead director.