Documents filed with the Securities and Exchange Commission show Icahn and affiliated funds bought 7.1 million shares for $118.1 million between Aug. 23 and Thursday.

Navistar shares jumped 8.6 percent in aftermarket trading.

The filing says that the activist investor believed the shares were undervalued. Navistar shares are down 46 percent since hitting a high of $71.49 in April.

Navistar shares gained $3.32, or 8.6 percent, to $42 in aftermarket trading Thursday. The stock added 12 cents to $38.68 during the regular session.

The filing says Icahn has had discussions with Navistar management over adding board members, but no agreements have been reached.

Navistar is based in Warrenville, Illinois.