A police spokeswoman said a suspicious homemade device was found after police stopped and searched the 4WD on the Princes Highway, near Ambrie Road, in Noble Park North about 4.30pm on Tuesday.
A quantity of drugs was also found in the car, the spokeswoman said. The driver, a woman believed to be in her 20s, is assisting police.
The area has been cordoned off.
All eastbound lanes of the Princes Highway are closed near Corrigan Road.