Twin 1999 454 MPI/Bravo 1. Everything was working fine Saturday. After docking for two hours at lunch the starboard engine would start, but would immediately stall. My sonic has two fuel tanks, so I tried using both tanks on engine with same results. After idling back to my marina I changed the fuel filter. Still has problem. The next morning I tried to start it, but still the same problem.

Any ideas for trouble shooting will be appreciated. The next thing I will try tomorrow after work is to bleed off any air via valve near injection rails.

Oh yeah, when the key is turned to the "on" position the fuel pump does kick-in.

vapor lock find the fuel presure test fitting under the front of the plunim sp. and let the air out of the fuel system andit should start

Rule out the ignition switch before you drive yourself crazy.

Does it quit as soon as you let off the key or does it stumble a bit before quitting?

good point but i have seen this motor vapor lock more then once but i would check the switch for sure

Sometimes it will run for a few seconds and slowly die out. The more people I talk to the more I think it's vapor lock. I'll bleed the fuel pressure fitting tonight and let yawl know how it goes.

Thanks bunches!

Mine was full of water and was doing the same thing. Was there water in the fuel/water separator when you changed it? I had to have about 3~4 gallons of water/fuel sucked out of my tank before it would run again.

Can't be vapor lock if it does it after sitting over night.
Does the Merc MPI fuel system have a return line to the tank?
If so it shouldn't have vapor lock. Not with a constant flow of fuel.

How about the fuel pump safety cut-off switch? Does it have one?
Some safeties have a start bypass to provide fuel pressure while cranking.

I went down during lunch break to look for the fuel pressure test fitting. I pushed it in and air did come out and then a couple drops of gas. Just to compare I did the same for the port engine and nothing came out of that one. Then I realized my boat keys were at home ...dadgumit! I'll try to start it tomorrow morning.

MLitefan, I added gas treatment and filled both tanks to the top during winterizing, so hopefully they are not full of water.

Iggy, I'm not sure if the merc MPI has fuel recirculation. However, when I open the tank access hatch I only see two hose connections per tank. Each hose connects to a different left/right/off switch-over valve. Also, I don't know if the fuel pump has a start bypass. How do I check for that?

Thanks all for the input.

I have been reading about this problem and think I have a similar one. One of my 502's starts hard after operating temp. Starts fine cold. The way I see it the return line from the presssure regulator goes back to the VST. Seems to me that could put allot of hot fuel in there. I have read some about options with cool fuel but not sure about the applications. I have fuel pressure gages ordered for mine so I can compare one to the other. Next question would be if it is a vapor lock condition how do you stop it? Somewhere I read there may be a service bullitin on this. If anyone knows let us know.

Thanks.

I'm guessing the MPI engines use an electric fuel pump right?
To see if there's a return line, follow the fuel line on the engine. If it has a return you should see a pressure regulator and a seperate line comming from the engine. Two fuel lines per engine.
You say you only saw two hoses attached to the fuel selector switch? The return lines could be connected to the main refueling hose.

If it does have an electric pump you should hear it turn on when you turn the key to the start position. Once the engine is running the safety switch, which may be controlled by an oil pressure switch or a sensor in the ignition system, will take over and provide power to the pump.
Find the fuel pumps, have someone start the engine while you listen or feel to see if the pump is running. If the pump stops when the key is released back to "RUN" position the trouble is in the safety switch.

BTW I have no knowlage of the MPI systems. I'm only giving you what I know of automotive engines.
Marine EFI, MPI, systems (even carburated engines with electric pumps) should have safety switches to prevent fuel flow if the engine is not running.