Top of piston stuck in #1 cylinder/ misfire
My 1997 Bonneville had 77,100 miles when I was driving home one evening. The engine was warmed-up and I got on the highway to go home. After a few miles the engine developed a misfire, then 1 mile more the check engine lite came on, I pulled to the side of the road and discover the oil level was low. I put oil in the car and drove home with the check engine lite still on and the engine misfiring. The next day my mechanic discovered the top of the piston was stuck in the #1 cylinder. The rest of the piston discinagrated and put a hole in the oil pan.
I have since put a new motor in my Bonneville but, the reason the engine developed this problem was related to the plastic manifold. Newer model 3800 series 2 motors have a thicker plastic intake manifold. The only problem I had with the car was occassional misfire at acceleration. My mechanic told me he saw evidence of oxygen leakage in the manifold which was responsible for the engine damage.
It may be a good idea to replace your old manifold with a new one if you like your older 3800 series 2 motor.