After securing a new exhaust and air pump, we looked at messy stuff: all the vacuum hoses, valves, and switches. The vacuum hoses are a very primitive computer that controls your engine’s emissions. It does so by noticing the vacuum pulled as engine rpm increases and the heat of the engine, and then opens and closes valves accordingly. For example, the Exhaust Gas Recirculation (EGR) valve opens and allows exhaust gases to reenter the combustion process to reduce the compression temperatures and thus reduce nitrogen oxides. Because EGR valves cycle exhaust gases, they often become dirty and need cleaning or replacing.