My starboard 496 mag ho has recently developed a single beep followed with the engine cutting out or completely stalling whether at idle or at cruise. I have read other threads that indicate the possibility of a bad ECM (infamous PCM 555). I changed the IAC just to be safe however it appeared to check out fine originally with my Diacom software based on it's duty cycle being between 15% and 30% at idle. After changing the IAC, I went ahead and risked swapping the ECM's from the starboard to the port and ran them. The problem did not chase the ECM (a relief) but the engine still stalled and this time stranded me with one engine. However, it did fire up at home once on the hose but then died again. I had done a recording on my Diacom the day before and slowed down the playback and noticed that the engine power was fluctuating between 6%-100% which I believe indicates a few things like water temp sensors and water pressure sensor. I did notice that as it would switch to 6% the water pressure would drop from 3 psi to about 1.5 psi which is too low. Note these are brand new impellers and wear plates in stainless steel Hardin Marine sea pumps. Another check later while running both of my engines they both display 3% water pressure. I am thinking that the starboard engine's water pressure sensor is bad....anyone got any insight or if I'm headed the right direction. I did pull that pump and checked the impeller and it looks brand new so I pulled the sensor. Is there a possible check for this sensor. I went ahead and ordered a new one from Merc and hope it fixes my problem.

One beep and it misses or dies, power fluctuating between 6 to 100%

If 12V power, then a bad connection or main power relay

Thanks for the tip AllDodge. I'll double check all of my battery connections. I would think that if I had both engines running on the same battery it would affect both. Can you tell me where the "main power" relay is located? I know I have a couple mounted on the bracket with the PCM but I am unsure what they are for. I do not have digital throttles or shift.

Please don’t run EFI engines (especially, but not related to) on the same battery as each other.

Oh I agree SB. I switched my port engine to the same battery I normally run my starboard engine (that one that's actin' like a fool) to see if it's something between that battery and the engine but I can rule that out. I keep engines separated when on the water.

Sorry, just noticed this
The MPR is next to the fuel pump relay and they are the same type and can be swapped

Look for wiring colors. The MPR will be the one with Red/Purple connections

I just wanted to give an update on what I found, figured out and repaired so as to help someone that has similar issues in the future. This forum is awesome and everyone has really great input. Anyway, after changing the water pressure sensor (it was fine), the MPR relay, it was fine, switching computers from port to starboard (not the greatest idea but I rolled the dice) and checking all connections, I read somewhere that someone had a similar issue with engine shutting off at speed or idle with no warning and after cooling down would start back up. Mine had recently started exhibiting a part that was apparently suffering from some form of heat saturation and I had read about this person changing out the crankshaft position sensor because there were apparently an issue with marine 496's as well as their automotive counterparts. I went ahead and bought one, installed and now my engine stalling problem has ceased. Thanks for everyone's input as I learned much more about this 496 mag ho by following the above steps. My only question though is this, why is the automotive version of the 2002 496 out of a 2002 Chevy 2500 8.1 liter $69.00 and the mercruiser verson $391.00? It seems to me there would not be any need to "marinize" a part that goes 8" down into the engine. The automotive version had also apparently been updated changing the dual O-ring to a single. I know it's a no no to use automotive parts in a marine engine but it seems to me this part really can't be "marinized" so to speak. I'd be curious if anyone has any input on this in case I need to go ahead and cough up the extra cheddar for this part.

There isn't any difference IMO, only things that cause an issue is finding it on a place like ebay or amazon and not a GM actual part. GM uses Delphi, Standard, Mallory and others, but some of the stuff found sites are knock offs. There is another part number that SB found and I updated the list on my other computer for the 2001 but haven't updated this one yet

2001 - Standard PC948, GM 12575182 NLA, Merc 883513 NLA, Block 1XA, 1XW
2002-2003 - GM 12575183, Standard PC 949, Merc 881666 NLA, Block 2XA, 2XW, 3XA, 3XW
2004+ - GM 12591720, Standard PC260, AC 2133826, Merc 892615002, Block 4XA 0M667238 & up, Block 4XW 0M672041 & up
CDI E13-0004. OEM#: Mercruiser 892615, 892615001, 892615002, Pleasurecraft R020035A
Replaces Part#: Mallory 9-29821 Sierra 18-7646

Woops I listed cam sensors not crank

^^^Those are camshaft sensor part #'s.

As far as Crank Sensors for the 496/8.1's ACDelco lists this:
https://parts-catalog.acdelco.com/ca...log_search.php
Engine Crankshaft Position Sensor
Part Number: 213-1556
Product Notes:
Engine Crankshaft Position Sensor
All; Incls ONE UPPER O-RINGS SEAL
Per Vehicle: 1; Years: 2001-2006