On July 1, 1998 Pontiac Bonneville with 3800 V6 overheated. Took it into shop and had cooling system checked. No pressure leaks were detected. When I left the shop, engine began missing and check engine light came on. Engine smoothed out, but took back to shop and they put on machine. Cylinders 1 and 6 had been misfiring, but seemed to be running fine. The next day on way to work, the engine began missing badly again and temperature gauge went up to about 240. After work, engine would not turn over at all. After having car towed back to shop, I learned that the upper plenum for the intake manifold, which is made of plastic, had become warped where the egr valve connects to the engine block. This allowed antifreeze to fill up the engine cylinders. The small repair shop where my car was repaired stated that they had replaced 10 of this particular upper intake plenum for this particular engine.

- Columbia City, IN, USA