A leak detecting apparatus is provided that tests for leaks in closed systems such as the air intake system of an internal combustion engine. The system connects to a conventional air supply that provides air under pressure. The air from the air supply is routed through a fluid tank where the air is directed against the fluid thereby generating fog. The fluid is preferably a mineral oil. The fog is routed out of the apparatus and is fed into the system for detecting leaks. Leaks are observed visually when the fog escapes from cracks or fissures in the system being tested. In order to aid in the observation of the fog escaping from the leaks, a light reflective pigment is added to the fluid thereby making the fog easier to observe upon the application of white light to the fog.
