7.4 MPI fuel issuse?
 

Hello, got the boat out today, fired right up, let it idle for about 10 min, got the wife and kids on and headed out, idled out of the cove and brought it up to about 2500 rpm, ran for about 5 min the started to fall on its face then died, boat would not restart, so I crank 4 or 5 times nothing, not even trying to start, so I pull the hatch up and start looking at at distributor and wires don't see anything wrong, so I have the wife turn the key, I hear the fuel pump, so don't think there is a issues there, I let it set for a few min and try starting again, nothing, I then turn the key on 5 or 6 times and the the fuel pump kick it's self on then wait for it to shut off, bam fires right up, I turn around to get back home around 3000 rpms and it runs again for a couple of min then starts to fall on its face then dies. I once again open up the hatch, see no leaks, I leave the hatch up, and start cycling the key on and off after about the 3rd or 4th time I can hear a pitch change from the fuel pump, I cycle it a few more times, then try it, bam starts right up, I finally make if into the cove at an idle and to the boat ramp, and just as I get to the ramp, it acts like it's starving for fuel and dies.

I changed the water/fuel separator a couple of weeks ago, so don't think that's the problem, unless it just now sucked some junk out the bottom of the tank, tank was completely full, just filled up today, was also the frist time out this year, I'm leaning towards a fuel pump or regulator issue, anyone have any ideas or other places to start looking, thanks for any and all help.

Re: 7.4 MPI fuel issuse?
 

Check the fuel pump relay.

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Re: 7.4 MPI fuel issuse?
 

Ok, thanks

Re: 7.4 MPI fuel issuse?
 

If you are getting it to cut out/die on you again I would see if there is still fuel at the shrader valve atop the fuel rail. You could also get a screw on fuel pressure gauge and run that on the boat to see what pressures you are getting. Though this does not exactly sound like a vapor lock issue I have had issues with my fuel cooler. Mainly after running and then shutting down for a bit. The fuel get really hot in the fuel rail and it will not start till it all cools down.

Re: 7.4 MPI fuel issuse?
 

What is the correct fuel pressure, when I first turn the key 42psi, engine running a constant 34psi, the engine was only at idle, I thought the pressure was supposed to be closer to 38psi. It ran 30min today on muffs no issues at all, but it was also only at a idle. I have changed the water separator again, refilled with fuel and bleed the air out, I also had to change out a faulty impeller, didn't realize it was bad, until I got home after it quit, put it on the muffs, and didn't have any water flow.

Re: 7.4 MPI fuel issuse?
 

According to the Mercruiser service manual, the correct pressure depends on which fuel system you have on your 7.4MPI. If it is an MEFI 2 system then the correct fuel pressure is 34 - 38 psi. If it is an MEFI 3 system then the correct fuel pressure is 41 - 45 psi. Check the serial number on your engine, if it is 0L01003-0L085013 then you have the MEFI 2 system. If it is 0L085014-and above then you have the MEFI 3 system.

Re: 7.4 MPI fuel issuse?
 

Thank you sir, mine is the MEFI 3, serial number IS OL644582, looks like I'm a little low on pressure, thanks again

Re: 7.4 MPI fuel issuse?
 

Sounds like you will need to follow the troubleshooting chart in the Service Manual. It could be the pressure regulator, vacuum lines to the regulator, the fuel pump itself, wiring or the relay. Happy hunting.