I have a 92 190E2.6 that has a slight stumble/roughness at idle that started 15k miles ago. The car has 75k miles now. I have changed the plugs with no change. Car runs fine and gas milage is good(24 to 28). No computer codes and egr is closed and not leaking. Idle speed is normal(700 rpm) and fuel pressure is 75psi and EHA current is at -3 to -5 ma when warmed up(I have sensor plate adjusted richer for a delta of -2ma from factory setup for faster starting @40k miles). I had replaced o2 sensor at 50k for surging at idle(cured). Any ideas of how to clear up this slight stumble would be apprediated.