Let's talk EFI Fuel Pressure Issues

[Liberator*21]

Here's what I've got..........

Merc 250 EFI/Brucato SVS/Bosch 044 EFI Fuel Pump, fresh injectors running 100% pre-mix (no VST), Mallory EFI Pressure Reg. w/Vac Boost Port hooked up to the intake.

Here's my issue.........

EFI Regulator at idle is set to 38-40 PSI w/vacuum boost port connected. The pump is very capable of pushing 75 PSI @ 80 GPH minimum, why is it that my fuel pressure drops off 8 PSI at WOT..................? All fuel line is -6 hose (3/8 ID) and new.

[flabum1017]

how's your'e voltage at the pump?

[Liberator*21]

That's a darn good question...........I really need to wire it direct to the battery next time out and see if the pressure stays.

Thanks for asking, I'd have never thought of looking there first.........

[mrcrsr]

i'd be curious to know how you have it wired, always use a relay, you can come off the battery for main relay power w/ an inline fuse holder, luckily you haven't blown it up, also, what do you have for filters? alot of people don't realize that there are 2 different merc spin on filter/separator elements, one is a 10 micron, one is a 20 micron which is what you want.

[Liberator*21]

Charlie,

This just started. I've run before with the cowl off and didn't notice a pressure change. I'm wired with a relay, fused but not drawing directly from the battery. Previously the boat had a Holley Pump in it when I bought it and when I changed it out to a Bosch I just used the same power supply wire and relay. I'll have to back track the wiring to find the source of the power. As for a filter, it's a NAPA GOLD water seperating filter new this year. I'll look at it this weekend and see what's what..........thanks.

[Bruster]

How is the check valve in the fuel tank? I've had several this year that were "stuck" and limited the fuel to the motor under WOT. Removed the valve, cleaned it, done.

[Liberator*21]

No check valve on this set up.......Tank-Filter-Pump-Fuel Rail-EFI Reg.-Back To Tank

[hydrostream1]

Is the pump pulling the fuel through the water sep filter?Do you have a pre pump feeding the high press bosch pump?Those Efi pumps are not designed to pull fuel,it has to be fed fuel at the bottom of the tank,which is hard to do in a marine application,so you have to have a low press pump with some kind of air separator like the stock VST tank.

[Liberator*21]

No pre pump. The BOSCH is mounted well below the bottom of the tank, and let me state again, this problem just started, Fuel pump set up has always been the same.

[YELLOWSS]

hey gary i would say the pump is going out. i have had them hold 60 lbs at a idle. drop the hammer and the pressure falls off by 15 lbs when they start going bad.

[BOATBUFF27@AOL.COM]

As the pumps get a lot of hrs. on them they will fall off a couple lbs. We found that routing the return line back to the "in" port on the fuel & water seperator it will help hold pressure up. It also keeps the seperator full of fuel, so no air gets in the fuel feed system. Been running this set up since 1988 with no issues. I also use an automotive in tank low pressure to feed the other "in" port on the seperator. Works great.

[ChrisCarsonMarine]

Boatbuff, should air inter your system from whatever source where does it end up? If you route your return line to the filter body any air that gets in the system seems doomed to just re-circulate through the fuel system and never return to the tank,what am I missing?Chris

[mrcrsr]

As the pumps get a lot of hrs. on them they will fall off a couple lbs. We found that routing the return line back to the "in" port on the fuel & water seperator it will help hold pressure up. It also keeps the seperator full of fuel, so no air gets in the fuel feed system. Been running this set up since 1988 with no issues. I also use an automotive in tank low pressure to feed the other "in" port on the seperator. Works great.

as chris said, definetly not the way to plumb the fuel system, trapped air has no way of getting out other then thru the injectors

[johnnyracer]

You have to run a return back to the tank from the filter seperator to rig like that and even then its not a 100 % air free but it will work on a consumer type engine.

[BOATBUFF27@AOL.COM]

Don't know if I am lucky or what, but with the in tank pre pump, to the in port on water seperator, out line to the Bosch high pressure pump, to motor. Fuel return to the in on the water seperator. Holds 39lbs. of pressure and has been running the 2.4 EFI high performance motor 7400 rpm since 1988. Whats running the return line to the tank going to do for me??