I have the same engine. I mentioned in a recent posting that we had a hose fail that runs from the forward end of the engine on the starboard side around to the aft end. This particular hose passes under the exhaust manifold and connects to a fitting on the port side at the very rear of the engine. In the process it goes up over a small metal tube, and ours had kinked and split right at the end there. It was a very flimsy hose, and we replaced it with a much more substantial fuel hose. Our symptoms were similar to yours - but we didn't realize what the issue was until the engine started smoking from a stream of water that was spraying onto the hot manifold and we had to get a tow.
As was suggested, take a good look at those hoses. In our case, the mechanic I called in simply had me pour fresh water into the tank while he watched to see where the leak was coming from.
