Well after hooking it up to a scanner, there were no alarms. Swapped high pressure fuel pumps and the problem followed. New pump should be here this week. Thanks again everyone for your input

Edit: JRider get your ass to work and off OSO

Same thing happened to mine, from Ethanol. My fuel pump had a rattle to it when I removed it.

Is there a trick getting access to the pumps? Mine is pretty tight & need to swap for troubleshooting also

I remember just loosening a few bracket bolts and sliding it out.

I suggest looking up your engine(s) on www.mercruiserparts.com to see brackets and schematics of the front of your engine as there have been a few different pulley and bracket designs on the 496s since their introduction, What may work on one, does not even exist on another.

There are two fuel pumps on the 496. One is the Boost pump and typically primary culprit on the starboard side behind the separator filter assembly, and then there's the main pump on the port side buried deep (you should hope to not have to change that one)....Depending upon the year, you should only have to remove the fuel separator filter and mount (two bolts) and lay it out of the way to get full access. Should be a spacer in there too to keep an eye on. The newer motors with a different separator assembly I "think" may have the pump built into the filter assembly? Not 100% sure about those editions. After that pretty simple and obvious disconnecting/reconnecting the lines AND the electrical connector. While you're in there good opportunity to look around for touchups or to tighten hose clamps etc.

Another indicator would be to hook up a fuel pressure gauge to the Shrader valve on the fuel rail up top and check for consistent fuel pressure. With no alarms you're most likely on the right path.

Another note on spark plug wires or dead coils....Should one of these happen you will most likely start seeing a lot of excess soot buildup on your transom from the misfiring or non-firing cylinder.

Good Luck!