There are 2 major problems with Porsche 996 and 997 engines: IMF failure and D-chunk. IMF or intermediate shaft bearing is a sealed bearing on the shaft on the flywheel. If the bearing wears out, the timing chains come loose also pieces of the bearing disintegrate and wreak havoc on the engine. This was not my problem, but is fairly common. My problem was d-chunk.The middle cylinder walls on the 3.4 liter and the 2.5 liter engine are weak and poorly made and vibrations causes cracks to occur in the cylinder wall. The cracks lead to a chunk of the cylinder wall to break off. The chunk is d-shaped, hence the name.You can imagine the problem that occurs when a chunk of metal starts flying around inside your engine.