350 mag mpi low fuel pressure.

[Vortec]

Hello everyone. I know here is tons of similar topics ,but i could not find my fix. Ok 1999 350 mag mpi has a steaady 18 psi of fuel pressure at rail engine off (should be around 30 psi). new filter,New pump, replaced fuel pressure regulator twice (OEM, pink mark)no change, pressure goes above 80 psi with blocked return (so no pump issues) checked pressure before fuel rail - no change, tried feed pump from a gas can before fuel filter - no change. Have used 3 different fuel pressure gauges. I ran out of ideas, PLEASE HELP
Can fuel cooler cause pressure drop?? I don't see any fuel leakage.

 

[Bt Doctur]

What is the rail pressure with the engine running with the fuel reg connected to vac and not connected to vac
With the engine off it should pressurize to atleast 32psi

 

[alldodge]

What are your symptoms other then fuel pressure?

 

[Vortec]

Hello, Thanks for replying. Pressure at rail with engine at idle is 15 PSI with vacuum, 18 no vacuum. Symptoms are no power under load and rpm don't go above 4000.

 

[alldodge]

OK what is your serial number?

 

[alldodge]

Also have you pulled the plugs to see if they are wet?

 

[Vortec]

serial L305046 . Plugs are OK. Boat starts and runs good except has no power at load like pulling a tube.

 

[alldodge]

Your fuel systems should be like this


This being the case, fuel pump will exceed 80 psi when blocked off
Fuel pressure regulator is set at 43 psi @ 1800 rpm, 38 psi minimum @ idle
Regulator is new so regulation should be as specified

Only thing left is injector(s) are leaking lowering pressure.

Plugs are not wet and will idle fine. So unless something is not hooked up correctly, you have a problem I have never seen,

Hey Fun Times have any suggestions?

 

[Fun Times]

tried feed pump from a gas can before fuel filter.

Did you use just a hose or an actual pump of some sort? One test to consider trying is to use like an outboard primer fuel bulb to try and build up pressure by hand to the whole fuel system through the engine to see if it helps with the engine running.
​Also ensure you are getting full 12.4+ volts to the fuel pump power system.

See if this topic may be able to help you out at all, http://forums.iboats.com/forum/engin...pressure-issue

Can fuel cooler cause pressure drop?? I don't see any fuel leakage.

This is just an unlikely theory.. But say if the fuel cooler tube was to be damaged by freeze damage, then there would be a possibility of fuel leaking into the engines water cooling passages...In-turn lowering fuel pressure but you should be able to see it exiting through the exhaust water system leaving a trail of fuel slick lines either in the lake or while running the engine on the garden hose.

If the little screen that sits under the fuel pressure regulator became clogged up with debris, then the fuel pressure becomes to high as it is part of the return system heading back to the filter assembly.

Make sure hoses between the filter assembly to the pump assembly are not kinked or damaged including internally as a hose with splinters to the rubber inside would affect fuel flow causing bubbles which could cause low fuel pressure. Also if the hose fittings and O-rings at the pump and regulator assembly are compromised/damaged then you'd have low pressure. Be sure the fittings at the pump and regulator aren't clogged up.

Are you getting a warring horn letting you know you have low fuel psi? If it gets to low "your" engine model should have a sensor letting you know it's low. See item number 14 for example, https://www.perfprotech.com/mercury...sn-0l304600-up/assembly/2717/135?umodel=10250

 

[chrisambrico]

I realize this post is old, but interested if you ever found cause. Having similar issue, however mine has power till around 3000 rpm. Fuel pressure is 25 with vacuum, over 30 without