Is this a tank vent problem?
 

I've been dealing with this same issue all summer long....99 454 mag mpi, some days it just runs absouletly perfect, then the next day I can be running & it will just fall off back to idle. The engine won't die, it will just drop back to idle, then stumble like it is running out of fuel. Sometimes I can let is sit for a min or two & all is good, however this past weekend it seemed to get worse. I've gone thru the entire fuel system, new regulator, pump, even the cooler is new, filter. I've replaced every sensor, except the TPS....it tested perfect. I've had the ECM reflashed, & of course a full tune-up, there are no codes. I've also checked all my grounds & they are tight & no corrision. There is just nothing consistent when it happens, I don't know when it happens, it just happens. It really doesn't make any sense at all to me, it goes against all troubleshooting methods.
One thing I have noticed, when fueling, & the tank gets close to full, I have fuel blow out the vent. I'm wondering if there is some fuel remaining in the vent line, & at times it will prevent the tank from venting properly.....I don't know if this is just a long shot, but I'm ready to try anyting. I contacted the tank manfacture & asked them about the pick up tube, I tried to remove it to check for any cracks, & I couldn't get it out all the way....they told me I can remove the screen off the bottom, & it will come right out, & it did....also, there is no anti-shipon valve.
I really am convinced that this is something "simple" I just have to find what "simple" is, it runs too good for this to be something major....

Rangerrick63, Back to the pick up tube, were you able to remove it completely? It is a common to have problems there. They will often crack where the meet the elbow on top. I'd check it like it was a peace pipe and make sure it holds pressure. Its possible something got lodged in the turn on top. I have even found a piece of brown paper bag that would randomly find it way to clog the tube. Does it happen more often when the tank is low? If your vent passes fuel when you fill I would think its OK. Good luck
Jim

Had a similar problem on a '99 5.7EFI. Turned out to be a sensor in the distributor.

Thanks for the input, Yes, I was able to completely remove the pick up tube. The tank manfacture says to leave it off, they don't intsall them in the new tanks. The tube seems to be fine, but I will look at it even closer this evening....
It does it with a full or half tank, or anywhere inbetween....no consistency what so ever, that is what makes this so darn tough to figure out. The reason I was thiniking possibly the vent...I read several posts saying that sometimes the boat builder will leave a "loop" in the hose, & I was just thinking that maybe at times the fuel settles there enough to restrict the venting.....Like I said, probably a long shot....

I've not ventured into the distributer, mainly due to the fact that the engine doesn't die, it just drops back to idle, & then stumbles when trying to plane off again...It's only after I let is sit for a short while that it will run good again.

My uncle had the same exact problem, no codes at all. Took it too mechanic after mechanic, no one could fix it. Finally the last mechanic (isn't always the last guy) said I've seen this before. Bad plug wires were screwing up the ecm. We all went "huh????" but the problem was gone.

Fuel Vent
 

Check your fuel vent line. Take it off the tank, and replace if access is not a problem. (Has the boat been sitting for any length of time?) Otherwise go right to the pickup: greenchainsaw:

Water in the fuel? If you think it is the vent then remove the fill cap and see if it runs better.

Jon

That is exactly how my boat acted up. It was the sensor assembly in the distributor. Merc had a TSB out at the time that addressed the problem. It stated that if the sensor had a green stripe on it that it was OK but if it had a grey stripe on it that it could cause those symptoms and to replace it (I may have the colors backwards...it has been a few years). Well, mine had the color that was good so the dealership wouldn't replace it. A regional Merc rep came down from WI. and spent a day on the boat until he finally replaced the supposedly good sensor and the problem went away. Problem here is that the 5.7EFI doesn't share the same distributor as your 454 MAG. If you could hunt down another ignition module for your distibutor you could try swaping it out. However, it is my experience that those are either good or they don't work at all and the ignition is dead.

Has anyone seen this problem on a 496 HO ?

We are chasing down a similar behavior......but.....we have isolated it to fuel pressure which drops way off when the boat hits 3600-3700 rpms. The boat has has 2 saddle tanks and an IMCO fuel tank selector switch so we have a bunch of trouble shooting ahead on this one. We have also seen fuel lines collapsing internally from ETHANOL making them soft and deteriorating. Not to mention all the issues mentioned above to check as well.