I would not keep it at all. The carbon buildup in the intake side and ports alone causes efficiency loss and power loss as the engine ages. When I still had the EGR on my Express van with the 350 Vortec I played with the EGR in the tune on the black box. I got 1/2 mpg gain disabling the EGR in the tune. You could feel the EGR turn on just off idle with the stock tune and gave the engine a slight hesitation. With it off it made more torque and accelerated better from a stop with less throttle and less rpms. I even disabled the EGR until 65 mph and saw no benifit even at steady cruising speeds. With the EGR active it always took more throttle to run down the road.
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