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2004 350 Mag MEFI4 fuel pump questions

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ob-1

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I am helping a friend work on his boat, 2004 with a 350 Mag MEFI4 motor, fuel pressure at idle is 35 psi, at 2500 it drops to 32 psi.
Manual says it should be 42 psi, how do I test the pumps to see which one is malfunctioning?
I don't want to replace both pumps if only one is bad, is there a way to test the transfer (low pressure) pump separately from the high pressure injection pump?
 
Docksidemarineservices said:
Your engine is not a MEFI...It is a Motorola 555 ECM.....


Check the regulator.......
Click to expand...

Regulator, fuel rails, and injectors are all brand new installed 2-weeks ago.
Back to my original question, is there a way to test the pumps individually to isolate which one is failing or, is it just replace them and hope you guessed correctly?
 
If you have a bravo drive you have a gen 3 cooler.... Pull the cooler remove the lid and you can separate the pumps to test.

Put a vacuum gauge on the fuel inlet, power up the cooler with jumpers and see if it is pulling better than 7 inches of vacuum... If yes, low pressure is good..... Make sure the cooler water passages are open. No water flow will hurt the high pressure pump.
 
the low pressure pump only supplies the fuel, the high pressure is what brings it up to operating pressure. also, are you aware that if this is an alpha set up, there are 2 regulators, 1 is on top of the cooler.
 
ob-1 said:
Regulator, fuel rails, and injectors are all brand new installed 2-weeks ago.
Back to my original question, is there a way to test the pumps individually to isolate which one is failing or, is it just replace them and hope you guessed correctly?
Click to expand...


The regulator on the fuel rail does nothing. It is dead ended.... Mercruiser does not use a return fuel line. The only regulator on a Bravo is on top of the Gen3 Cooler.... Is this the one you replaced?

If it is an Alpha Drive, you have a Gen2 cooler and that is located on the port engine mount studs... It also only has one regulator, the one on the rail is the same as the Bravo, useless.

BTW, yes the non functioning regulator on the rail has a vacuum line, it is there in case the non used regulator ruptures and will direct the leaking fuel to the intake.
 
Its a Bravo 3 Duo-Prop, I believe that both regulators were replaced, the shop where the boat was purchased just replaced the injectors, fuel rails, and regulator under a Mercruiser warranty/recall. Thanks for the info about the vacuum gauge I will check that tomorrow!
 
There is only one regulator..... Read my post above....


If the shop did the gen3 cooler repair, you may have gotten a complete cooler or you may have gotten all the parts minus the pumps...

My guess is you got a new cooler housing, lid, regulator and the moved the old pumps into the new cooler housing... If this is the case, there is a problem with the reassembly of the parts. Particularly the seal on top of the HP fuel pump.....
 
They did not change the fuel module or the pumps, I am now thinking they only replaced the fuel rail mounted regulator not the one on top of the fuel module. My neighbor has elected to replace both pumps and the regulator hoping this will solve the problem, I am not much for the "scatter gun" approach but its his Nickle so........
 
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