EXHAUST GAS RECIRCULATION SYSTEM (EGR)
The egr valve is shown opposite. It is a water cooled tube with a valve in it, connecting the exhaust manifold to the inlet manifold. (Not water cooled on pre 1999 engines) The egr actuator is controlled by a solenoid valve fitted to the bulkhead and connected to the brake servo line. This is pulsed 50times per second by the ecu, with the 'on time' being varied to modulate the volume of exhaust gas fed into the inlet manifold. To calculate how much exhaust gas is being recirculated, the ecu monitors the MAF sensor and notes by how much the signal drops when it opens the valve. (Exhaust gas replacing the intake air causes the drop in MAF signal) This is the only reason for the pre 1999 engines having a MAF!
The ecu keeps the egr valve closed under acceleration, (or boost pressure would escape through the valve into the exhaust manifold) so theoretically there should be no need to disconnect or blank it off. However, there is no harm in disconnecting the small bore pipe to see if it makes a difference.
The solenoid valve buzzes when working - this is normal, though sometimes the EGR valve itself makes a metallic rattling / tinkling noise due to it chattering on its seat when partially open, This can be irritating - if so just try disconnect the pipe.
Bosch claims that up to 40% recirculated exhaust gas reduces emissions, improves fuel economy and minimises soot production.
Egr removal stops the "dirty" exhaust gases being recirculated back through the intake and into the chambers. The whole system was simply fitted by manufacturers to help reduce NO emissions from the vehicle, but it affects its performance as well.
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