Abstract: A system for measuring an automotive engine exhaust constituent includes a meter for measuring the mass of air flowing through the engine and for generating a flow signal corresponding to the airflow, and a sample handling apparatus for separating a fraction of the exhaust flowing through the engine and for conducting the exhaust fraction to an analyzer. The system further includes diluent adding means for doping the extracted exhaust fraction with a diluent gas upstream of the analyzer and a processor for receiving the mass flow signal from the engine mass flow meter and for operating the diluent adding device in response to the flow signal. The processor operates the diluent valve such that the flow rate of exhaust gas into the analyzer remains a relatively constant fraction of the total exhaust flowing from the engine.