You can try swapping the props from side to side(run shifter in reverse for test) and see if issue follows. That would take the props out if the mix.

Props or drive are not the issue as the problem occurs in throttle only mode as well.

Apologizes, should have taken more time to really read the issue

The does it IN THROTTLE ONLY MODE is the give away that it needs to be re-calibrated

How is this done?...via a certified tech and merc computer?...which by me is unfortunately hard to come by.

I'm also wondering about the fuel trim. I know auto engines adjust but on these non-cat EFIs are there any fuel trim adjustments with no O2 sensors?...my understanding is there is no verification. The MAP along with everything else can provide the PCM with info to set the injectors but will it make adjustments based on burn? If there is a long term fuel trim adjustment I could see that being an issue. Trying to get a better understanding.

Dealers and others with "I think" the Merc G3 computer can Cal. Do a search on line and there are several companies

With out )2 sensors, the PCM just uses the Fuel tables programmed into it. If the MAP, TPS, ECT or fuel pressure changes, then the PCM only works with what its provided

Thanks. That's kinda what I thought about fuel trim. With no burn verification it can't adjust long term...or short for that matter. I will try and get a tech with Merc computer to the boat. I just wonder why it would go out of calibration all of the sudden? For the previous 3 plus years of owning the boat the RPM were dead on unitl the end of last season.

If anyone has any other ideas in the mean time I might be able to try I'm listening.

Where are you located ? out of all the things you have swapped I didnt see the MAT sensor listed, could that cause a flux in rpm's ?

Boat is located in Lorain, OH on Lake Erie. There are dealers/repair shops out west but probably would have to get the boat to them and who knows how long they would want if for. Sucks losing time during our short season. The boat does run and can be used so I'm trying to troubleshoot without having it at a shop for an extended period. Hopefully I can get someone with the Merc computer to come out to the dock.

I don't believe I have a MAT sensor. If I do could you please elaborate on where that would be. I did swap the MAP sensors which resulted in no change.

I thought all EFI had them but Im no 496 cat...........If there is one it would be screwed into the intake plenum, manifold absolute temperature MAT

Believe its out of Cal from how its acting. Doing a re-Cal might show that one of the sensors is out of spec so its telling the PCM incorrect info. Drive by wire is great until it doesn't work as we all know, Hope there are others with more smarts then me