Mercruiser 350 Mag MPI Low Fuel Pressure

[jk184]

Thought id share my issue and solution. Bought a 98 2200i about two years I thought it ran well but I was only getting about 2600 rpm. Until this year when I was maxing out around 2000. Changed fuel filter, fuel pump, cap and rotor, and tps. That stuff helped but only got me up to 2400. Finally dug into the plug wires and noticed they were all completely wrong. Cant believe it would run at all like that, but now I have them in the right position im getting around 3800rpm. Feels like a completely different boat

[Evan0177]

Also checked voltage to the fuel pump and had 12.37

[Evan0177]

I’m having the same issue with my 2000 mercruiser mpi gen+. I have a constant 22 psi at idle. 33 psi with the vacuum line off and 43 with the return line pinched. I have replaced the fuel pump, fuel pressure regulator, anti siphon valve. I have tried an external tank. I put a new fuel line in from the tank to the fuel filter. I blew out the high pressure lines and rail. I put new spark plugs, wires, distributor cap and rotor. I still have 22 psi after doing all that. Mine stays at 22 psi after turning off for a long while. My boat won’t get on plane. Tops out at around 10mph, around 1400 rpm. No power at all past 1400 rpm. I’m out of ideas if anyone has any would appreciate it…

[Evan0177]

Boat: 2001 2100V. Twice in the last few times out the engine completely died while under power. When attempting to restart, it would crank and not fire. At this point, I would open the engine cover, pump the throttle and it would fire back up. I also removed the flame arrester thinking this was chocking it out even though I just recently cleaned it. After much research on these forums, I bought a fuel pressure gauge and it's reading about 25psi and drops to 20-21 when rev'ed up to about 2500rpms. According to my research, this should be 32psi. The pressure holds steady after killing the engine, so I ruled out stuck injectors. I replaced the fuel filter and anti-siphon valve with no change in pressure. I haven't lake tested it yet, but it runs fine in the driveway. Any ideas on the cause of my low fuel pressure and do you think this is directly related to my engine completely dying under way? I'm assuming my next step is replacing the fuel pump and pressure regulator.

[Bretlee]

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[boatwakes]

Well played sir!!! That's an anomaly if ever there was. Good job!!!

[Cr8zyMatty]

Finally found the problem! Took the boat to the shop, as I mentioned. They were no help- said that everything tested as good as a brand new boat. I started thinking about it- and had already checked inlet air, verified fuel, and verified spark. I also checked any switch/sensor that could give a limp mode. So what else could it be? The ONLY issue I could see is an exhaust restriction.

I picked up the boat and pulled the whole system. I thought it could have been a flapper wedged shut in the riser. (It was not). So I kept pulling parts out. Turns out that my muffler has a moving float/baffle inside That had cracked in half and wedged itself in a way the restricted nearly 3/4 of the exhaust flow out of the boat. I couldn’t believe it! Can’t make this stuff up. So I took a 3” hole saw and cut through the baffle on each outlet. Took it to the lake and the boat ran perfect. We will eventually replace the muffler but for now it’s fine. There is a small increase in exhaust sound at lower speeds but barely noticeable.

So glad she’s up and running great again! Wish I had thought to start the whole process by checking my muffler fluid, would have saved weeks of diagnosis.

[Cr8zyMatty]

Was finally able to get the boat on the water again. Replaced the distributor cap and rotor, along with spark plug wires and checking continuity on those couple of switches. Everything went good in the driveway, but same problem once we got on the water. I’m at a loss now, planning to take it into the shop and go from there. So much for a Labor Day weekend on the water now!

[boatwakes]

!

[Cr8zyMatty]

Glad you located what seems like most of your problem!

I’m waiting on the distributor cap, rotor, and wires to come in to try on mine. (New plugs in already) While waiting, I’ve been reading through the service manual #24. I discovered that the “alarm” (buzzer, horn, whatever you want to call it) is mounted but both leads are cut. Also learned that despite all my gauges reading normal operating, there are also switches for various items that *might* interrupt normal operations? Although I’ve never seen high temps, low oil levels, or low pressures (aside from the initial low fuel rail)- I’m wondering if maybe I have a bad switch in the system. I have not found a specific readout in the service manual that states “holds engine at XXXX Rpm” it does state it would interfere with normal operation. I would think I’d see a trouble code if a switch was shot but I haven’t seen any. I’m planning to at least check the Transmission temp switch (normally open) for continuity, as it is the only switch on my 350 black scorpion called out in the wiring diagram. I also need to somehow figure out how to wire back in the alarm buzzer- it could tell me something I’ve been missing! Any other thoughts on things to check when I replace the ignition items listed above?

[Fabricator909]

Mmmm, you could tell there was a splice in the original fuel line for something.When did they start adding the boost pump?
It runs guess, It'll be getting a full wiring job this winter.

[Fabricator909]

Just curious if your boat has the boost fuel pump? I know my 21v the previous owner put the boost pump on a switch, why, who the hell knows, but the boat would get on plan, then start starving for fuel. It would idle fine, rev in neutral,it would also go about 10-15, but under load it ran out of fuel quickly. I'm not sure where the original is supposed to be, but mine was on the starboard side of the v drive.

[boatwakes]

Ahh yes, the old "Hey, where's this wire go?" problem. Glad you got it!!!

[chopper515]

Update on my issue:
Took the boat to Table Rock Lake for a week and the problem continued. I threw on a new distributor cap, rotor, and coil wire.... still died occasionally but would always eventually start. When I got home I replaced the ignition sensor (3 wire unit) and coil. The existing sensor was the 3 wire unit but the ground was not hooked up. It was still grounded by a wire attached to the bottom bolt as it comes through the distributor. Took the boat out Saturday and the boat didn't die all day. It did develop a little stutter/hesitation while surfing with the ballast full. I have plugs and wires coming Friday.

[freeheel4life]

Could be weak spark/misfires. I would just plan on doing cap, rotor and plugs. You'd be amazed to find how often its still factory cap on some boats. If you look close at the cap and can see a tape line in the black paint just below the posts its a factory cap.