Diesel Exhaust Gas RECIRCULATION: Description

NOTE: The following information applies to the 5.7L Diesel engine. The 1.8L. Diesel does not have an EGR system.

Two basic Exhaust Gas Recirculation (EGR) systems are used on General Motors diesel engines to lower oxides of nitrogen (NOx) emissions. Federal vehicles use a modulated (internal) EGR system, while California vehicles use a throttle position modified orifice (external) EGR system. See Fig 1.

Fig 1: General Motors Diesel Engine EGR Valves

Courtesy of GENERAL MOTORS COMPANY

Both systems introduce exhaust gas into the intake manifold and then the combustion chambers. This recirculation lowers combustion chamber temperatures and NOx emissions. EGR valves allow exhaust gas flow into the intake manifold at all times, except wide open throttle.

Federal models use an Exhaust Pressure Regulator (EPR) valve in the exhaust system to increase back pressure in the exhaust flow. Some Federal models (not Eldorado, Seville or Toronado) have an additional EGR control assembly between the vacuum regulator valve and EGR valve to reduce recirculation when the Torque Converter Clutch (TCC) is engaged.