Thanks, that what I thought - the inner pipe should carry the exhaust gas and the outer pipe is just a heat shield. Where my outer ribbed pipe is damaged I can see thru to the inner pipe, and it looks to be solid. The damage was there when I bought the car several years ago, and it looks like it was eaten away by acid or something. I didn't notice it at the time when I bought it, but this car has always had a rough idle since I've owned it. No EGR error code, though.
A few days ago I patched the outer ribbed pipe with a piece of a steel sheet rolled over it and clamped on, and sealed the gaps with thermo-steel but the rough idle is still present.
I gues it'* time to get a replacement EGR pipe assembly.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles