1998 Chevy Lumina EGR codes

Usually an EGR malfunction is not in the valve itself, which as you've seen is just a vacuum diaphram and pintle. Most often, it's restriction of flow caused by the gunk that forms naturally in the intake manifold clogging up the EGR passage within it. If your codes include PO401, insufficient EGR flow, I would definitely look into that. Examine the intake port where the EGR bolts on and make sure you can pass a soft wire through the passage or passages. May require removing the intake for cleaning in severe cases. Secondarily, most EGRs use a sensor or rely on a computer signal to monitor engine backpressure and adjust the EGR operation accordingly, and that mechanism may fail.