I don't know where to start with this issue, a lot of moving parts to the story.


Short story, picked up a boat last fall with a reman 496HO that ran for about 10 minutes and seized up due to no/low oil. The guy didn't want to put more money into it, so sold it as is. What a steal I got it at! I had the 496 rebuilt with new bearings and pistons, assembled long block by the machine shop. I then had my mechanic assemble the rest of the intake, efi, etc. and install it in the boat.


After installed, we had a rough idle where it would want to die unless bumping the throttle. I took it out for a run and the boat died on me, it was in gear at the time and being DTS (Digigal Throttle/Shift), the ECM still thought it was in gear and left me stranded.


Dropped it off at the shop to troubleshoot the rough idle. So far, they've identified 3 or 8 injectors were too small and replaced all 8. Initially, they thought the fuel pump was bad/low voltage and replaced that. The original 496 was a MAG and the paperwork I found on the motor was an HO, so we shipped the ECM to be tuned for HO versus MAG.


As of now, the boat idles good and no codes, but he said it has no power and won't rev above 3500. He's going to test coils under load this morning, but I think he's out of ideas at this point and thinks the cam timing may be off.


Before I have the engine pulled for a second time and spend who knows how much more money, any ideas on what the heck could be going on here?

Have they checked fuel pressure under load? Any misfires at 3500 rpms? I once had a 5.7 merc that got rebuilt but could not spin more than 4000 rpms. Turned out the gear ratio in the drive was incorrect

Is the alarm going off at all?

The 496 can go in limp mode for multiple reasons and limit rpm/power. You would need to scan ecu to see if it is in guardian mode.

Fixed! Corroded coil wire was gounding out. Shop was out of ideas and was thinking something internal possible after a fresh rebuild.

540Fever,

It's always the simple sh!t, ain't it.....

Thanks. Brad.

No doubt! Hidden right in plain sight.