The gasket under the plenum failed and coolant began leaking into the cylinders. I had two independent mechanics tell me that with the engine leaking that much coolant into the cylinders, and the resulting hydro lock (engine wouldn't even turn over) I would be better off replacing the entire engine. Both estimated a cost around $3500-$4000 for the job. Fortunately, I was talked into simply replacing the plenum and gasket myself and moving on. That only cost me $350 and a couple long days in the garage. Now, 35,000 miles later, my Bonneville has over 200,000 miles on it, but it is beginning to lose coolant and I suspect the defective plenum design has struck again.