A system for detecting engine fluid constituents includes an engine having a sample channel having a conduit for a working engine fluid. The system includes an electromagnetic (EM) source that emits EM radiation through a first metal tube, where the EM radiation is EM energy at a wavelength of interest. The system further includes an EM detector that receives a remainder radiation through a second metal tube, the remainder radiation including the remaining EM radiation after passing through the sample channel. The system includes a controller that determines a composition indicator signal representative of an amount of a constituent in the working engine fluid in response to a strength of the remainder radiation, and determines a concentration of a component of interest according to the composition indicator signal.