Mercruiser 5.0 MPI voltage fades to the cool fuel module, seeking advice

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Searay 2005 Sundeck Serial Number OW063935. We have replaced the cool fuel module and the sea water pump, both are now new. When we put a gauge on the fuel rail we have 5 lbs of pressure. If we put a bulb on it and pump up pressure, it will run but slowly goes down and dies. Stumped!! Any suggestions? There is fuel in the line. Been working at this for three weeks and buying parts. Today replaced all the o-rings in the lines. Still haven't solved the problem.

The first thing we replaced was the Parker fuel solenoid valve as it was too hot to touch and we thought that was it. We replaced the whole cool fuel module gen III, not just the pump. Then put new o rings on everything thinking it was an o ring problem. The line that powers the pump is getting fuel. When we bypass the system and run power directly to the pump, it works fine. When we check the voltage path to the pump when you turn the key on, it gets 12 volts and then immediately fades back to zero. Why is it not getting voltage??:smt021

 

[Bt Doctur]

When we bypass the system and run power directly to the pump, it works fine.

Do you have the correct fuel pressure this way?
bad ground or bad relay contacts,
Personally I would remove all the extra parts in the fuel system and install manual shut-off`s on the tank feeds.

 

[Bt Doctur]

from Man 24

 

[paulswagelock]

If you "Hotwire" the pump and everything works just fine, then the troubleshooting is on the circuits that energize the pump. Bad relay, nicked wiring harness, maybe others. Need the wiring diagram and permissive list for that circuit.

 

[jima]

There are two elect. Relays on top of the engine. One is for the ecm, and the other is for the fuel pump.
Get it from Merc dealer and try that.