Known as "EGR".

Fitted to Diesel Townaces with automatic transmission to reduce NOX (various oxides of nitrogen) emissions. They can be produced during "over-run" when ordinary air, which is about 20% oxygen and 80% nitrogen, without fuel enters the combustion chambers while they are still very hot. The oxygen in the air combines with the nitrogen in the air instead of with hydrogen and carbon in fuel.

Works by diverting some of the exhaust gas through a metal pipe, about 25mm diameter, around the back of the valve cover into the inlet manifold. A flap valve where that pipe goes into the inlet manifold is opened by a vacuum servo which is controlled by an electric signal applied to a solenoid-operated vacuum valve next to it.

General opinion is that in a 10-year old car it does more harm than good - causing a lot of carbon soot to build up in the inlet manifold and in the EGR system itself, possibly even preventing the flap valve closing - therefore allowing some exhaust gas to recirculate in "normal" driving. Blank off the metal pipe to ensure that it is disabled without relying on the flap valve being fully closed.

Here is Jim Adgo's sketch for drilling the EGR blanking plates. One method is to leave the system in place and slip in one thin one at either position. Otherwise remove the system and you will need some different M8 bolts and both plates, which will need to be thicker. Jim suggests 7mm steel but this is not critical so long as one face is flat, nor is the overall shape which can be rectangular - as a guide inlet: 68x32mm, exhaust: 66x38mm overall.