Exhaust gas recirculation (EGR) is a system that takes exhaust gas from a diesel’s exhaust manifold and reintroduces it into the intake of the engine to reduce the combustion chamber temperature.
*See below for latest update on EGR
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine’s exhaust gas back to the engine cylinders.
Here is a video and a few general (non-Bongo specific) articles on the subject:
Some owners fit an EGR blanking plate. Miles per gallon can improve slightly, but sometimes this action can fail the emissions and smoke test. Some people remove and blank off hoses. Some simply remove the vacuum hose, rather than blanking off.
*It should be mentioned, that as of May 2018, the new MOT may fail any vehicle where the EGR has been tampered with, removed, or blanked. Allan Bugg has written more about this here. (Scroll down his page to MAZDA BONGO EGR SYSTEM). Allan told me that: “The legal option now will be to clean it. Not much point in fitting a replacement as you’d have to remove yours to fit the replacement, so may as well clean it while you’re there.”