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Message: <blockquote data-quote="shovel" data-source="post: 1528549" data-attributes="member: 29855">In addition to the EGR system there is also the thermactor (AKA secondary air, AKA "smog pump")  system which is vacuum operated.  Most people choose to delete the thermactor system as it has really nothing to do with the engine running it only exists to supply oxygen to the catalytic converters so they can do their job.    The EGR system is not in any way related to the thermactor system but also uses vacuum and a solenoid mounted under the pass. fender to work.    EGR has quite a lot to do with how the engine runs;  if it's on when it's not supposed to be your engine will run terribly and if it isn't on when expected it will also run poorly.     EGR's job is to maintain dynamic compression ratio under part throttle for better fuel efficiency & it also regulates combustion temperature for reduced emissions .   Because EGR affects volumetric efficiency the fuel and spark strategy are heavily dependent on it so you can't just unplug it/block it off and not tell the ECU.  For either of those systems if they are not maintained in working order they should be deleted cleanly, both are kinda all-or-nothing systems.   Having them each work properly is ideal and of course in some places legally required - but if you're going to delete either of them it must be done completely and cleanly for the engine to work well on the stock ECU.  Owning one of these pre-OBD cars is a challenge;   if you've got budget for it I strongly recommend finding a factory service manual it's worth its weight in gold.</blockquote>