Agreed as I said fuel related issue. If you notice any internal corrosion that is a sign of moisture / water in the fuel side. . It is always wise to do fuel samples and when boat lay up over 30 days - one needs to protect the fuel and fuel system. Soup mix as it is called in the industry.

And I did mention sometimes 496 pistons just let go - again not forged items in the 496's.

Always scan the pcm before the engine rebuild to find anything underlying..496's are very problematic with sensors that can become an issue period - The deal with scanning the engine is just because one owns a scan tool means nothing because 8 out of 10 do not know how to indepth read the scan info nor know what to look for. - A scan is not going to show fuel pressures but will show oil pressure, ECT, manifold temps, etc, sensor readings and so on. . Many think just because the scan shows no fault codes that everything is fine --- wrong. Each reading has a specific parameter and needs to be within its correct parameters,

Industrial 496's are not ran in the same environment nor ran at the on off / high low rpms as the marine versions . Nor do they sit for long periods of times. SO fuel is always moved thru them and fresh fuel is more constant on the industrial version 496's. But I bet they do share some of the same common internal issues. Weak pistons and rods and IMO prob weak Valve springs over time.

Water in the fuel can show signs on the spark plugs as well.

I agree with all the above info. Definately check fuel psi when you start new engine. Stock KOEO should be roughly 42-3 psi. Keeping monitoring fuel psi and scan engine as you load test on the water. Member Rage here had a faulty fuel psi regulator and chunked a forged Mahle 496 piston. If the fuel system is not correct it does not matter what pistons are installed.

If you look at the huge number of 496s produced for marine apps the stock failure rate is actually pretty small. Early grueling development tests, including the “Marine Dock” test that involves running the engine at full throttle for 55 minutes and then idling if for five minutes, 300 times shows its capable of handling some abuse if all is correct.

Koeo was 40 when I unplugged the vacuum line it went to 45 psi I'm going to run it with the gauge on today to monitor. I removed the fuel pressure regulator and the paint inside was bubbled up but all intact so I removed all the lose paint. Will have the engine scanned next week put s half hour on the engine with the muffs on last night so going to take it in the water today. Thank you for the advice !