1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

[grizer]

Changed the Fuel Pumps and the problem persists. Any other suggestions? Anyone with late 90s Merc 350s w MPI had anything happen? Thanks in advance!

[darrenlife]

I read through the post and didn’t see confirmation that your batteries were checked. Albeit, it would be strange for both starting batteries to fail at the same time. Also double check charging circuit breakers are open, good grounds, etc.

[grizer]

@darrenlife The batteries are relatively new, they crank the engines nice and strong. I'm not sure why you are asking about the batteries but am giving that insight back for your info. Besides, once the boat is running, the batteries don't have much to do with things do they? In other words, the boat is then primarily running off Alternator (using a car reference I know). Do the fuel pumps and battery charger circuits have something directly to do with the fuel and throttle systems that I am not thinking about?

[grizer]

Related to the main topic but does a "master schematic" exist for a 350 mariner? I'm trying to find and figure out how all these different systems might tie together. People have said "check the batter voltage" and "check the sync and tach gauges" BUT that implies that there is some connection between each of those items and throttle, fuel pump inputs, ECM inputs etc. Trying to see how/where that is all documented. Anyone know?

[km1125]

When you said you replaced fuel filters, did you replace them ALL? Are there two sets for each engine?

[bud37]

The interesting fact here is that you say both engines are the same, they both run fine etc, etc....so consider what is common to both, something electrical is my guess.......now as has been pointed out earlier the ignitions can be tied together at the sync gauge......so one idea,...... batteries is another area...a bad fill up is another......obviously something happened to cause this.

I really think you may have to bite the bullet here and have someone hook up a laptop to the engines and take it out for a run to get a recorded log to inspect all the engine parameters while under Load.....that cannot be duplicated running at the dock, that just leaves you open to damage.

[tomschauer]

OK, if they both start and run fine up to 3k, and you have at least 12-14v on the volt meters with the engines running its not a voltage issue.
Do you keep the boat in the water all year? If so, maybe you have a nice carpet growth on the bottom. You would be amazed at how that can and will slow you down.
I had a 29 crownline with twin 5.7 mags and B3 drives, a 54mph boat, was unable to use it for two months one year due to work travel, it wouldn't get above 30mph. Stopped at a shallow beach area and found 3" shag carpet on the bottom. Scraped the worst, got 40mph, after an hour cruise 50+.

[tomschauer]

The above assumes you have the proper fuel pressure 35 psi plus. If not, did you change the regulators or just the pumps? And did you replace the very small $25 fuel filters in the cool fuel housing?
Then there are the injectors. But if you don't have the proper pressure to them, I wouldn't change them until you do.

Unless you have the mercury smart craft system, which I doubt on a '98, the sync will not restrict the rpms, only monitor them. Without the smart craft, there is no relation to the fuel pumps, throttles etc. The ecm, turns on the fuel pump (again, its on or off, not variable speed or variable voltage) and controls the fuel injectors based on the throttle position.

[grizer]

All new fuel filters, replaced fuel pumps, stil 20-25 psi at fuel rail and still limited to about 2600-2800 rpm. No change. Merc suggested fuel regulator test by pulling vac hose at intake but that produced no different results. Really baffling and my mechanic just through up his hands.

[km1125]

Does each engine pull fuel from its own fuel tank? Are there any switches involved? Is there a possibility they're both pulling from the same tank and that tank (like an anti-siphon valve) has an issue??

On an MPI engine, usually if there's a vacuum line on the fuel pressure regulator the fuel pressure will be lower when the engine is under low load (like idle or higher RPMs in neutral) and only let it go to full pressure when there is no or low manifold vacuum. The vacuum line "tells" the pressure regulator to lower the pressure. As more load is placed on the engine and vacuum goes from ~15 to ~5 (or ~0), then the pressure regulator increases the pressure. You can think about it like the injector has to "push" fuel against a pressure instead of the fuel being "sucked" out of the injector with some vacuum. This is so the injectors see the same delta in pressure from the inside (the fuel pressure) to the intake manifold, so every time they squirt fuel for "x" milliseconds they know exactly how much fuel was squirted. Even though YOU see the pressure changing when you measure it, the idea is that the fuel injectors DON'T see a change in pressure. You're measuring fuel pressure relative to air pressure (14.7PSI absolute) but the fuel injector "sees" the delta between the fuel rail pressure and the manifold pressure, which can change from nearly 0 absolute (theoretically) to 14.7 absolute. At idle, the manifold pressure is about 7.5 PSI absolute, so if your target fuel pressure is 40PSI, then at idle you'd only see 32.5 PSI, and these are the corresponding differences you should see when you yank the vacuum line off the pressure regulator while running at idle.