On my isuzu 2.8TDI Ive two EGR valves, they where not fitted to the earlier 2.8's in the troopers but where added by VX at a later date following a ruling by good ole Europe over emmisions, this recirculates used exhaust gases back into the engine to burn a gas ( cant remember the name) this is not tested for on the UK emmisions test.
This works well when the engine is new, but when the engine is older it can carbon up the inlets and it gets less efficiant and causes the car to smoke.
Ive blocked mine off to avoid this crud getting into my engine, had MOT's passed no problem.
Though some cars have the EGR mapped into the ECU so its not advisable to block them, but mine is a kind off bolt on idea.
Here is a piccy of my EGR valves when I blocked them, this stuff must act like grinding paste in the engine.