I have a 496 MAG HO twins. When I turn the key to the "ON" position the fuel pressure pimes and turns off on the starbord side and on the port side it builds up pressure but doesnt turn off and keeps running. Engines run fine and both have 43 PSI and A/F is good too. How can I fix this? Is there anything I can swap to test this? Is it a simple electronic part i can replace?

Kieth:

One of the relays at the ECM mounting bracket is a fuel pump relay which is switched on and off by the ECM. If the relay is sticking on (closed) it can allow the pump to stay on after the other relay on the other engine which is working correct shuts of the pump with the signal from the ecm. Sounds like you have a simple relay to replace and problem should disappear.

Best Regards,
Ray @ Raylar

I had this problem on one of my 2005 496's. The relay wasn't the problem. Sent the ECM back to Merc for new software and it solved the problem. This was covered by warranty (Merc authorized) at the time.