Engine Stalls when Throttled up

My father in law had the boat running for half the summer about 3 years ago, and then one time out he put it in the water and it would stall out every time he tried to throttle it up. He stored it for 3 years and now I am trying to fix it as he gave it to us as a present.

We took it out of storage after 3 years and drained all the fuel out of it. I put fresh fuel in it, injector cleaner, and a new fuel filter on it. It took about 15 - 20 attempts to start it, but then ran for about 5 minutes before stalling out. I changed the TPS sensor and it did nothing. We then changed the fuel pressure regulator, and now it runs like a champ on idle. Still takes 5 - 10 time to try to start it. We've had it running 3 times now on idle for about 15 minutes each in the driveway with mufs on it. Every time I go to throttle it up, the RPMs raise a little and then it wants to stall out on me. I have to quickly throw it back into neutral to prevent it from stalling. From the research I have done it sounds like it could be a fuel pump. It's the 1994 OMC Cobra 5.8L 351 EFI so it has two fuel pumps. One Low pressure in the front, and one High pressure on the top rear side of the engine. When I turn the key over to prime the pumps the rear one whines very loudly and if I put my hand on it, I can feel a lot of "action" in it. The front one makes no whine but I can still feel something happening in it. The rear high pressure one also whines quite loud the whole time the engine is on and idling.

My question in the end, is how do I know which fuel pump is bad? Is the High pressure one whining because it is not getting fuel and the low pressure one is bad, or does the whining mean it is bad and needs to be replaced? Or is there something else I am completely overlooking here?

Any help would be great guys. Thanks!!!

On the bright side, it starts and runs. That means everything is working. I suspect the fuel starvation is somewhere in the tank or the hoses. Unless you are running some kind of magical fuel from the 1990s in the tank, you have ethanol in the gas. That tends to erode the inside of the (probably original) fuel lines and gunk up the works. On the intake ends of the fuel pumps are wire screens. I imagine those are plugged up something good.
Time to clean the fuel supply including the tanks. Report back after you inspect the intake end of the lift pump. Buy and install new fuel line that is ethanol resistant.

When FIL was experiencing problems it was likely due to the ethanol fuel. So it's a simple, but tedious and spendy fix.

o2batsea said:

On the bright side, it starts and runs. That means everything is working. I suspect the fuel starvation is somewhere in the tank or the hoses. Unless you are running some kind of magical fuel from the 1990s in the tank, you have ethanol in the gas. That tends to erode the inside of the (probably original) fuel lines and gunk up the works. On the intake ends of the fuel pumps are wire screens. I imagine those are plugged up something good.
Time to clean the fuel supply including the tanks. Report back after you inspect the intake end of the lift pump. Buy and install new fuel line that is ethanol resistant.

When FIL was experiencing problems it was likely due to the ethanol fuel. So it's a simple, but tedious and spendy fix.

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Thanks for the advice. I'll try this tomorrow and report back on my findings. I sure hope it is something as simple as this.

So I removed the fuel line and the low pressure fuel pump. Screen on the pump is crystal clear. I blew air through it both ways just to make sure. Hooked up 12 volt to it direct and the pump purrs like a kitten. I am going to install a fuel pressure gauge right after the fuel pump and reinstall everything and hopefully that will rule out any problems before that unit. As soon as I confirm fuel pressure I will have no choice but to replace the high pressure pump at the back of the engine. Hopefully that is the problem as it shines quite loudly while the engine is running.

Fuel pressure is good after the low pressure pump. Starts first time now, and revs a bit more than it used to, however still quits after being throttled up after about 3000 RPM. Rear High pressure fuel pump still whines, so I am thinking that might be the problem. I'm trying to get a mechanic to come out and take a look at it before I spend $230 on a new pump that may not be the issue. Unless anyone else has some good suggestions, I'm out of options.

An in-line high pressure pump is NOT 230.00. Shop around.