My 97 Deville Concours (300hp Northstar engine) has developed some symptoms my local mechanic can't diagnose and don't show up on a scan for trouble codes. It has quite high miles (over 150K) but the compression is decent. When it's running, it seems to run fine. However, it has become a little hard to start and apparently won't pass smog in its current condition. It has the typical oil leaks (probably from the case halves?) but no other obvious signs of problems. Plugs and wires and all oxygen sensors have been changed relatively recently. Worst case, I've been thinking of rebuilding or replacing the engine, mostly because of the miles and oil leaks, and now, because of this problem. but I don't want to do that if that doesn't address the other symptoms. Any suggestions? My next step is to try the dealer.