Mercruiser 350 Mag MPI Low Fuel Pressure

[Cr8zyMatty]

I’ll be able to get back into diagnosing the boat in about a week, but my plan was to continue with the easy stuff (check distributor cap/rotor and potentially replace plug wires).

Is there any value in running through some sensors? (MAF/IAC, any others?) I find it odd that 2200 rpm is the consistent engine limit....

the engine idles fine and runs like a top, still revs high in the driveway just not on the lake under load. I give it more gas it it sounds “bogging” Wondering if maybe there is a limp mode interference or something?

[freeheel4life]

Seems like you've verified fuel and air arent your problems. So ita onto ignition and compression.

[Cr8zyMatty]

Spent a little time running through some things and lake testing the boat, here’s what I found (problem still exists)

-Replaced Spark plugs
-pulled anti-siphon valve (all good)
-blew fuel pickup back into the tank (all good)
-pulled flame arrestor and inspected (all good)
-blew out fuel tank vent line
-blew out fuel tank to filter line
-checked for any codes on the ecm (only showing code 12- system operational)
-checked alternator/batt voltage (13.7-13.9v running, batteries also good)

Here’s the weird part- I checked fuel psi at the rails while underway on the lake: only at idle do I see 25 psi. When giving gas (little bit or WOT) my psi holds above 30 (I saw 30-32).

However, the engine still will not go above approx 2200 rpm. What am I missing?

[Cr8zyMatty]

Tagging on to this thread- I’m having the same issues with my 1998 350 black scorpion (315 hp).

Boat is a 1998 Pre2200v w/t.

Replaced filter, pressure regulator, and pump. Still seeing about 22 psi at the fuel rail at idle and up to 1500 rpm (on a fake lake).

on prime the system gets to 30 Psi no problem, and the engine is not throwing any codes.

the boat ran fine for the last year, then one day after most of the day skiing and running around it just slowly dropped off plane and won’t get above 15-17 mph or 2200 rpms. Give it more throttle and it just bogs out (won’t die and fires up first crank). Pulled a fuel sample from the Bottom of the tank and it looks good, no gunk or water. Boat has just over 300 hrs, and was stored/ serviced meticulously by previous owners indoors and in an air conditioned space.

The line from tank to filter is good and tight, I have not pulled the anti siphon valve or pickup from the tank (guessing its brass and threaded into the pickup elbow at the tank) However, if I “siphon” gas out of the tank using just slight vacuum from my thumb gas comes right out no problem. I have not pulled the fuel lines from filter to pump. I have not pulled spark plugs, wires, distributor, or blown out the fuel tank vent line. I am doubting it’s any of those since we’ve been running the boat so often, but that may be incorrect thinking. Had it out 4 times this year, once every day for a week.

any help or advice is appreciated!!

[chopper515]

Just finished installing the pump and it runs MUCH better and is holding a steady 30 PSI. Before it would stutter and sometimes die when revving and dropping fast back to idle which I thought was the IAC valve but it's much smoother now with the new pump. I'm now running non-ethenol fuel whenever possible so we'll see how this goes.

Gumby, Just saw your post about insulating the lines after completing the job. I'll take this into account if I have further issues. What is the "newer 3 wire pickup" and where can I get it?

[chopper515]

I pretty much just dies. I'm putting in the new fuel pump tonight and I have a distributor cap and rotor coming this week.

[chopper515]

Had the boat out yesterday and stalling issue happened 3 more times. It just falls flat on its face while under power. Then it would crank without firing for a minute or two...wait a couple minutes and it would fire and run fine. After the 1st time it happed I thought it might be vapor lock so I ran it with the engine cover propped open and blower on all day but it died a couple more times after that. Any help would be appreciated.

[chopper515]

I run ethanol gas but I treat it with Star Tron Enzyme Fuel Treatment. That's interesting that you replaced the pump and still show somewhat of a low pressure. One more thing to note, my pressure jumps to +-30 psi what I turn the key and the pump cycles...then it settles to 25 psi.

[chopper515]

Gumby, Did you replace the pressure regulator too?

[chopper515]

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.