English sold cars used different fuel to Europe and during the unleaded changeover loads of vehicles had rough idle issue due to carbon in the exhaust [cold short trips]. Worldwide made Gm vehicles I worked on, and ford like transit often had EGR valve issues. Lots of reasons given for this most of the time a blocking plate would solve the cold start, air flow and idle problem. American cars are often different tune. I worked on a holden that had been reprogramed under warranty to suit Darwin and required to be reprogramed for NSW when they moved back here.
Few different cases to get tuning ideas from.
1 Rodeo would come in with the air conditioning clutch not working. the egr valve was a step motor and if it out of calibration it will emissions limp home mode-without the warning light. it is a step motor you cant remove it without an
ECU fix.
2 Brand new Hino O5 Truck over Christmas had a blocked EGR as it is uses to patch roads [very slowly]-the
ECU warning flashes for 32 hours then derates the engine. The code is a canbus communication fault. Often owners have no idea this has occurred. until you drive up a hill.
3 Navara with DPF. The
ecu has a routine of DPF burn as it drives along the
ecu will derate if it obtains the wrong sencor imputs.
4 old engines sold in OZ, often the same engine from japan etc without the EGR. at cruise throttle the
ECU/ Carby thinks the engine is under higher engine loads running lots of unburnt fuel. my charade required exhaust cleaning when E10 was used as cat would not fire up.