Low? Fuel Pressure 350 mag mpi

JimGuy341

I went down the rabbit hole of fuel pressure for a 2008 350 mag mpi engine (1A075xxx).  Mercury confirmed I should expect 38 psi at idle and 41-43psi at 1800+ rpm.  So I measured mine.  Twin engines, both showed 38 psi at idle and only 36 psi at 1800 rpm.  I asked Mercury if 41-43 psi is the expected value or just the max value and if there is an acceptable pressure range.  They said no, 41-43 is expected and I am low.  What's strange to me is why are both engines behaving the same?  Just coincidence?  Approx. 540 hrs on each.  Fuel system components are original as far as I know.

Before I go any further, I wanted to see what your thoughts are?  Am I looking at a fuel pressure issue on both engines or am I okay for my serial number and maybe there was a change in software/hardware boosting fuel pressure at a later date?

  

YYZRC

What symptoms are you having?

PickleRick

I'm not an efi expert but your ECU/hardware doesn't have any control over the fuel pressure.  Your high pressure pump puts out about 60 psi and your vacuum controlled fuel pressure regulator dumbs it down  to the 30-40ish range by sending the excess fuel back to the fuel filter.   (You 350 experts chime in if I'm wrong on the MPI)


There is a common issue with paint flaking off and clogging the fuel system on the bravo set ups... diesels don't have this issue which is why diesel hard tops are better!!!! 

If it was mine and I'm having no running issues I would assume my gauge and or sensor is a few psi off being both motors are identical.  

If it was my boat and I'm hitting the same speeds/rpms that I was when it was 500 hours newer, I would keep an eye on it but it wouldn't be an issue I'd feel the need to address.    But, if I was having performance, fuel consumption issues or running/starting issues then I'd dig into it starting with the potential of the paint flake.

It's my experience with an EFI system that isn't getting enough fuel pressure is they'll fall flat on their face when load/rpm is applied, if they even start at all. My experience with EFI is mostly two cylinders engines and they are not nearly as fun 

JimGuy341

YYZRC said:

What symptoms are you having?

I'm having trouble with rpm fluctuations on my port engine when on plane (no issues with stbd engine).  On plane, the rpm will start to drop, then I'll give more throttle, it'll come back for a few minutes then overshoot.  Then I'll pull back on the throttle, it will come down, hold steady for a couple of minutes them start to drop again.  Plugs/wires/cap&rotor were replaced 2 years ago.  Fuel filter new this year.  Off plane, rpm holds steady and idle is normal.  So this is what got me on to fuel pressure since it is a simple check.

YYZRC

I would swap the fuel rail/injectors between the two motors. It’s not a difficult project, you could probably do it in an hour. 

PickleRick

If both are doing giving the same issue I'd start with what both engines share.   Do they both use the same fuel tank pickup?  If so I'd start with borrowing a small outboard fuel can and run your fuel feed to the engines with that and see if your fuel pressure improves.   If you're lucky it might be an obstruction in the fuel feed hose from the tank to the motors or a fuel pickup issue.  

earl1z19

Did you remove the vacuum line from the regulater when checking the pressure at the rail? Did it change?

JimGuy341

PickleRick said:

If both are doing giving the same issue I'd start with what both engines share.   Do they both use the same fuel tank pickup?  If so I'd start with borrowing a small outboard fuel can and run your fuel feed to the engines with that and see if your fuel pressure improves.   If you're lucky it might be an obstruction in the fuel feed hose from the tank to the motors or a fuel pickup issue.  

Only happening with port engine.  They have separate tanks which is why I'm thinking fuel quality isn't the issue.  If I had bad fuel in the port tank I would expect it to be in the stbd as well.

JimGuy341

earl1z19 said:

Did you remove the vacuum line from the regulater when checking the pressure at the rail? Did it change?

Unfortunately I did not.  The line was pretty snug on the fuel pressure regulator and I didn't want to force anything.  And since both engines showed the same fuel pressures I just figured the regulator was fine.  Now I'm not so sure and I'll probably repeat the test.