Many vehicles suffer from EGR (Exhaust Gas Recirculation) problems. The EGR valve mixes exhaust gas (5 to 15 %) with the fresh air going into the engine, this reduces emissions (nitrogen oxide). The valve is computer controlled and should not be open at idle or full power, however the valve can stick open upsetting tick-over, general running and black smoke if diesel. The Renault Traffic etc.(also Vauxhall Vivaro) EGR valve disc can become detached. May put Engine Management light on, or flash Glow plug light if diesel.
Will normally need new valve, you can try cleaning or you can block the ports as temporary repair or to help diagnose the problem (with metal or gasket plate) if vehicle runs fine with ports blocked EGR is the fault (Engine Management light will still be on). See picture below, you can sometimes just loosen the two bolts and slide a piece of card to temporary block the ports when testing.
They can be electric or vacuum controlled, but most report back to the ECU so disconnecting won’t always put the Engine Management light out, but may help with engine running. Sometimes tapping the valve will make it close and work for a while.
Ford EGR Valve above.
The Renault (also Vauxhall Vivaro) EGR valve disc can become detached and jammed slightly open, see picture above. Note it is possible to turn valve to block the exhaust port and secure with one bolt as a temporary repair, make sure you remove the disc from port if detached.