Have Boat Questions? Ask a Boat Repair Expert.
You say they couldn't find anything wrong. Does it still only run 4600 rpm? What did they check, what did they do to the engine. Why did they give it back obviously not running correctly?
It is obvious it didn't go into Guardian mode.
I am assuming you have the correct spark plugs in the engine? What do they look like?
It's possible it could be as simple as a dirty or leaking injector.
What are your fuel and air pressures? Do you have gauges or did the dealer check? Are they fluctuating?
I would recommend taking it to another shop. Get a print out of not only the trouble codes but all the parameters of the engine. Have them check the injectors and pressures.
Can you take a picture of the spark plugs and post it here for me?