Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

I purchased an 02 Centurion knowing it had a "bogging" issue at 3k+ rpms once it was fully warmed up, although fairly intermittent. Tested the fuel pump out of the water, and a perfect 43 psi. Took it out for a water test today with a fuel pressure gauge hooked to the test port. I ran it for about 30 minutes, back and forth on the water, again a perfect 43 psi. After about 30 mins, it started bogging, and I checked the pressure. When the bogging was about to occur, I noticed a noticable 5 psi tick down in pressure. When it bogged, it dropped all the way to 10 psi. This 100% eliminated any other issues other than the fuel pump.


The mercruiser mag mpi 350 has 2 pumps. One in the fuel cooler (high pressure) and the priming pump (low pressure) at the fuel water separator. Im guessing the issue is with the high pressure, considering there is no issue at idle or in neutral; only under load and 3k+ rpms. Does anyone know exactly how the 2 fuel pump system works? Am I right here in thinking its the high pressure pump? I have a new one on the way. Has anyone replaced the fuel pump from the genII fuel cool system? I purchased the pump separate from the cooler.

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

I'm no expert with the cool fuel system but, the primary (1st fuel pump) just supplies fuel from the tank to the main (2nd fuel pump). While it is possiable for the main pump to go bad I'm thinking there may be a problem somewhere else. Maybe fuel pressure regulator, vaccum leak, electrical relay or connection. A pump just pumps, while it can mess up once warmed up it shouldn't go back to normal later. Fuel injectors are a very slim maybe, but not likely.

Like I said, I'm no expert just don't think it would be the pump.

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

fouroheight68 said:

...The mercruiser mag mpi 350 has 2 pumps. One in the fuel cooler (high pressure) and the priming pump (low pressure) at the fuel water separator. ...

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Not according to the Merc service manual for that engine. Only one pump. (See attached). The high pressure pump is the only pump. It takes fuel from the filter and supplies to the fuel rail. The pressure regulator just dumps back to the supply line to the pump... I'd be more incline to suspect the regulator...

Chris.....

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

achris said:

Not according to the Merc service manual for that engine. Only one pump. (See attached). The high pressure pump is the only pump. It takes fuel from the filter and supplies to the fuel rail. The pressure regulator just dumps back to the supply line to the pump... I'd be more incline to suspect the regulator...

QUOTE]

Told ya I wasn't an expert. My Merc 502 MPI uses a VST and a primary pump.

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Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

What's the Merc serial number of your engine? Sounds like you may have a straight inboard (not outdrive) with the fuel boost pump.

I don't think you have a pump problem, but it's very possible you could have a plugged or partially plugged up fuel system, possibly the fuel pickup tube or the antisiphon valve. I would sure be doing some testing before throwing parts at it. If you can't get fuel to the pumps, they sure can't build up the correct pressure or volume.
Try these tests before you buy pumps.
http://forums.iboats.com/showthread.php?t=283269

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

Don, yes its an onboard ski boat. I think I may have found the culprit. I traced the pump power wire to the starter selenoid where it was so loose it could easily spin on the post. Problem is, when I carefully torqued it down, the post snapped! Argh, new starter on the way now

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

Not sure why you would have a wire to the starter solenoid, even if you did, when the engine wasn't being cranked over it wouldn't have power to the pump. Assuming it was hooked to the D terminal in the image below.

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

fouroheight68,
Did your discovery really solve the problem? I am having the exact same issues, and I am so baffled as to if it is the fuel boost pump or the actual fuel pump/cooler system.
Fuel pressure at idle is about 40 psi, and when the boat gets up to about 2500 rpm, it will begin to spit and sputter and keep dropping fuel pressure down to 10 psi. When I pull off the throttle, it will ramp back up to 40 psi...then do it all over again...
I found debris in the inlet of the fuel boost pump...just not sure how the fuel pump and booster pump work in conjunction with each other.....

fouroheight68 said:

Re: Mercruiser Mag MPI 350 Fuel Pump(s) Replacement

Don, yes its an onboard ski boat. I think I may have found the culprit. I traced the pump power wire to the starter selenoid where it was so loose it could easily spin on the post. Problem is, when I carefully torqued it down, the post snapped! Argh, new starter on the way now

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Did this fix the problem. I was thinking mine was a plugged fuel pump filter. Does the fuel pump itself have a filter?

MileMarker74 said:

Did this fix the problem. I was thinking mine was a plugged fuel pump filter. Does the fuel pump itself have a filter?

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You're looking at an 8 year old thread. Please read the forum rules, especially about posting to threads beyond 3 MONTHS old.

Chris.
iboats mod team.