454 Mag Fuel Starvation. Now will not stay running

[HighRoller]

1999 Ultra 20XT. 454 Mag MPI/Bravo One. Stock except EMI headers and an AZ Speed/Marine Tune. 419 hours and meticulously maintained.

Boat has always run perfectly. Yesterday I pulled out of the cove, got on plane and was throttling up when the engine felt like it dropped a cylinder. Throttled back, it started running normally. At very limited throttle it runs okay, but a bit soft. If you throttle up it acts like it is running out of fuel after about 10 seconds. throttle back and it was fine again.

This morning I changed the water separator filter and backed it in the water. It started, then I heard the butterflies start opening as the engine started to struggle. I had already cycled the key so I didn't think it was the filter still filling. Shut it off, cycled the key, started it again, ran for five seconds and died. Now it barely starts and dies immediately.

I have fresh gas in it. Have switched tanks back and forth. Recently changed the oil and made sure it was not overfilled.

What am I missing here that would bring about a sudden fuel starvation issue that is getting worse? An air vent to the tanks or an inline valve in the fuel system that is clogged or malfunctioning?

Thanks in advance all.

 

[Shlbyntro]

did you prime the fuel filter when you changed it? also sounds like you may be losing a fuel pump. even prior to the filter change. gen2 cool fuel pumps are very prone to it. only way to verify is with a fuel pressure tester on the engine while it's happening but I can say with a 90% certainty that that's it.

Meec also has a service bulletin on the gen 2s, they want you to add a low pressure boost fuel pump on the big blocks. there is a kit for it

 

[HighRoller]

did you prime the fuel filter when you changed it? also sounds like you may be losing a fuel pump. even prior to the filter change. gen2 cool fuel pumps are very prone to it. only way to verify is with a fuel pressure tester on the engine while it's happening but I can say with a 90% certainty that that's it.

Meec also has a service bulletin on the gen 2s, they want you to add a low pressure boost fuel pump on the big blocks. there is a kit for it

I cycled the key 4-5 times in the garage after filter install. Fuel pump still sounds as strong as ever when it activates with the key. Got to the ramp and cycled it another 4-5 times. Previous owner says it sounds like it is still not primed. So I am headed back to the ramp to see if I can nurse it a bit and get the air out.

Thanks for the info on the Boost pump and fuel pump issues. My experience is that electric fuel pumps only die one way: SUDDENLY. Are these Merc pumps different?

 

[4Waters]

I'm pretty sure @Shlbyntro is in the right track but it also sounds like bad plugs, especially your original concern as you were throttling up.

 

[Icky]

I cycled the key 4-5 times in the garage after filter install. Fuel pump still sounds as strong as ever when it activates with the key. Got to the ramp and cycled it another 4-5 times. Previous owner says it sounds like it is still not primed. So I am headed back to the ramp to see if I can nurse it a bit and get the air out.

Thanks for the info on the Boost pump and fuel pump issues. My experience is that electric fuel pumps only die one way: SUDDENLY. Are these Merc pumps different?

Did you put fuel in the filter before you reinstalled? If you pull the fuel line off and cycle the key ( put fuel line into a container) does fuel come out?

 

[Shlbyntro]

I'm pretty sure @Shlbyntro is in the right track but it also sounds like bad plugs, especially your original concern as you were throttling up.

gotta fill the filter when you change it. but these electric pumps will often quit making sufficient fuel pressure as opposed to outright quitting, in fact it is very rare that they outright quit. It is a very difficult diag too because a lot of times they will be running fine for the mechanic and the only way to verify is to load test with some heat in the engine

 

[HighRoller]

Thanks everyone for the replies. Problem solved.

No, I did not fill the filter before installing. Oops. It is a small filter and I assumed an electric pump would fill it easily. So I obviously got air in the system that needed to be worked out.