OK, so I want to bring closure to this thread, for the benefit of those who offered me advice, as well as future boaters who might find themselves with a similar problem. After several incorrect diagnoses, a bunch of repairs, four launches to do on-the-water testing, and lots of $$$ spent, I am happy to report we have FINALLY found the root cause of my engine's power loss. In spite of losing a good deal ofthe prime boating season, I have to say I'm relieved the boat is fixed and on the positive side the experience has raised my knowledge of boat engines significantly.
As I last noted, the problem stumped two local mechanics and our strategy was to call in a rep from Mercury Marine to help diagnose. The Mercury rep brought a truck-mounted dynamometer so the engine could be run under load on dry land. The bottom line: the problem was an INTERMITTENT CrankshaftPosition Sensor (CPS). The CPS provided confusing data to the computer, which in turn didn't set the timing correctly. As I learned over this experience, the timing on this engine is completely set by the computer based on the CPS and camshaft sensor. There is no need or provision to adjust it by moving the distributor using a timing strobe light. The CPS on this engine, unlike other sensors, does not have a fault detection mechanism,which is why there was no error code. I'm told newer Mercruiser engines do have a CPS fault code.
The CPS was replaced, and I took the boat to the ramp to launch it for another test. We idled out of the marina, and once clear we throttled up, and finally, the RPM climbed up over 3000 and we popped up on plane. What a relief! We went on and off plane twice just to make sure it wasn't a fluke. It seemed to us that the boat got on plane easier than it ever had in the past. We'll see if this continues to be true on future excursions. My Floscan reported the fuel burn on plane (~20 kts) to be 13.5 GPH, which is normal based on my experience. So in conclusion, the boat is running perfectly at this point. Now we just have to wait for NEXT boating season!
I was curious why the CPS failed in the first place. The mechanic said that the CPS was partially "melted" which indicates that the engine overheated. Well, I did have an engine overheat 2 years ago. The alarm went off and I shut the engine down, and had to get towed back to the ramp. The mechanic at that time replaced a few things, including the thermostat, and thought he fixed it. On my next cruise it overheated again and I had to limp back to the dock. In the end it turned out to be clogged exhaust manifolds, which were then replaced. I guess between these two overheat scenarios, the CPS was damaged, but continued to operate OK but degraded over time until it finally hard failed last June. The overheat had also warped my heads, which was the reason the valve job was done a couple of months ago.
So there you have it. Once again I appreciate the advice you all offered. I feel a little bit smarter regarding boat engines, but I'd be perfectly happy if I never needed to diagnose another problem like this again!
As I last noted, the problem stumped two local mechanics and our strategy was to call in a rep from Mercury Marine to help diagnose. The Mercury rep brought a truck-mounted dynamometer so the engine could be run under load on dry land. The bottom line: the problem was an INTERMITTENT CrankshaftPosition Sensor (CPS). The CPS provided confusing data to the computer, which in turn didn't set the timing correctly. As I learned over this experience, the timing on this engine is completely set by the computer based on the CPS and camshaft sensor. There is no need or provision to adjust it by moving the distributor using a timing strobe light. The CPS on this engine, unlike other sensors, does not have a fault detection mechanism,which is why there was no error code. I'm told newer Mercruiser engines do have a CPS fault code.
The CPS was replaced, and I took the boat to the ramp to launch it for another test. We idled out of the marina, and once clear we throttled up, and finally, the RPM climbed up over 3000 and we popped up on plane. What a relief! We went on and off plane twice just to make sure it wasn't a fluke. It seemed to us that the boat got on plane easier than it ever had in the past. We'll see if this continues to be true on future excursions. My Floscan reported the fuel burn on plane (~20 kts) to be 13.5 GPH, which is normal based on my experience. So in conclusion, the boat is running perfectly at this point. Now we just have to wait for NEXT boating season!
I was curious why the CPS failed in the first place. The mechanic said that the CPS was partially "melted" which indicates that the engine overheated. Well, I did have an engine overheat 2 years ago. The alarm went off and I shut the engine down, and had to get towed back to the ramp. The mechanic at that time replaced a few things, including the thermostat, and thought he fixed it. On my next cruise it overheated again and I had to limp back to the dock. In the end it turned out to be clogged exhaust manifolds, which were then replaced. I guess between these two overheat scenarios, the CPS was damaged, but continued to operate OK but degraded over time until it finally hard failed last June. The overheat had also warped my heads, which was the reason the valve job was done a couple of months ago.
So there you have it. Once again I appreciate the advice you all offered. I feel a little bit smarter regarding boat engines, but I'd be perfectly happy if I never needed to diagnose another problem like this again!