EGR Valves

There are several types of EGR Valve, earlier systems use a vacuum-operated valve, whilst newer vehicles are electronically controlled.

Diesel High Pressured EGR Valves:

Divert The high-flow, high-soot exhaust gas before it enters the diesel particulate filter - the soot can combine with the oil vapor to create sludge. The gas is then passed back to the inlet manifold either via a pipe or internal drillings in the cylinder head. A secondary valve is also used to help create a vacuum in the inlet manifold as this is not naturally present on diesel engines.

Diesel Low Pressure EGR Valves:

Divert the exhaust gas after it has passed through the DPF. This gas has a lower flow but is almost completely clean of soot. The gas is then passed back to the inlet manifold via a pipe.

Gasoline EGR Valves:

Divert the exhaust gases, much like the high pressure diesel equivalent. The vacuum created by the cylinder depression, draws the exhaust gases in and the flow is regulated by the opening and closing of the EGR valve itself.

Digital EGR Valves:

Feature a solenoid or stepper motor and in most cases, a feedback sensor. These valves receive a pulse width modulated signal from the ECU, to regulate exhaust gas flow.

An EGR valve controls and regulates a proportion of the exhaust gas into the inlet of the engine. The main reason for this is to reduce Nitrogen Oxide (NOx) levels at the combustion stage. When the air/fuel mixture is burnt in the combustion chamber the formation of Nitrogen Oxide increases as the temperature rises.

Re-circulating a proportion of the exhaust gas back into the air/fuel mixture lowers the combustion temperature in the cylinder, reducing the rate of NOx formation. Many EGR valves now incorporate a cooler to decrease the temperature of the exhaust gas. Early designs use vacuum to operate the valve mechanism whereas most modern types are electronically operated by solenoids.

Exhaust flows through the valve and most of the time it is diverted through the cooler where the gas is cooled by the engine coolant. The cooler bypass (operated by a vacuum diaphragm) is only in the closed position when the engine is cold. This helps the get the engine up to the optimum operating temperature quickly.

The motor cam moves the re-circulation valve in or out to control how much exhaust gas is allowed back into the inlet. An electronic circuit board uses a positional sensor to provide the ECU with accurate feedback as to the position of the valve and the amount of of exhaust gas that is being re-circulated.

The most common failure is carbon build-up inside the EGR and cooler. This is inevitable over a long time period, but is significantly increased with city driving and short runs. Problems with the particulate filter can also cause excessive carbon build-up. Other problems are often related to mechanical wear. The signal fed back to the ECU about the valve position has to be accurate so any variations outside of the normal tolerance are detected by the ECU and error codes are then logged. The cooler bypass can also sometimes fail or develop external leaks.