I have a 2007 fountain lighting with the staggered 700SCi & #6 drive set up and have been having a problem for quite some time. The port side motor is good to go but the starboard side motor is basically cutting fuel to itself or in guardian mode when under load, in Forward, when it gets to around 1500ish and beyond. The tach, oil pressure and water pressure gauges are also going out of whack when this happens as well but they’re reading perfect on my garmin/vessel view on the dash so I doubt it’s any of those sensors. Both engines turn over, start up and idle perfect. In neutral when both engines rev up they both hit around 2800-3000rpm before the rev limiter on the computer beeps a few times and cuts them out. So why would the boat beep and cut fuel under load at the 1500ish rpm range and not till 3000ish rpm in neutral? What changes when the boat goes into gear? Why would the boat be in guardian mode and not throw any codes? (I had them scanned)

I just recently put brand new interstate cranking batteries in the boat and voltage is good, tps sensors are good, I replaced both iac’s a few weeks ago so those are good. Pcms are good. Grounds to batteries and backs of engines are good. Wiring to alternator is good. Coolant level is good. Water pressure from the sea strainers is good. Engine temps are good. I have all new oil, oil filters, water separators, in line fuel filters, air filters as of 5 hours ago so those are good. Im running out of ideas here but I’m thinking it’s a electrical wiring/ground issue.

Any insight is helpful. Thanks.

Need to check fuel pressure under load. Revving to 3000rpms in neutral does require the same amount of fuel as being run under a load.

Maybe your starboard PCM thinks you are sill in neutral? Over speed doesn't set a fault, just sounds the alarm rev limiter. Connect a scanner and make sure the PCM sees that your starboard engine isn't showing that you are in neutral when you shift into forward.

It's odd that your gauges are going crazy so I'd also try disconnecting your AGI box and see if the problem goes away. They are notorious for failing and maybe it is messing with your tach signal and causing the PCM to activate the rev limiter. Your Garmin / VesselView signals are not routed through the AGI box, which could explain why they are working properly.

GF?

^^ Great suggestions, Also did you install Mercury brand IACs with the factory gasket? To throw this in the mix, you could have a PCM that needs repair.

Yes they’re factory mercury IAC’s so the computers should be fine. I bought them at a local marina that’s a merc racing dealer. They weren’t bad when I changed them, I just figured I’d do it as PM as I didn’t know the last time they were changed.

i disconnected the agi box and nothing happened but we were thinking the same thing, that it might have something to do with the over speed and pcm thinking the boat is in neutral when it’s in gear. I’m going to try and get a scanner on it in the next few days when I’m on the water to get some real time data.

Alright so today I went down to the boat and put a new AGI box on the side that was giving me problems (starboard) with the tach, oil pressure and water temp and those are all back to normal. My gauges started messing up immediately after I filled the boat up with gas with the keys in the on position but not running, so maybe that messed up the agi box? I’ve seen it happen in older trucks that when the key is on and you’re fueling up it basically knocks out the instrument cluster so I’m not surprised to see this happen.

Also, the engine is not in guardian mode, it’s basically sounds like a super quick blow off valve like the motor is losing spark so possible a bad coil or something else? Plugs are brand new but wires and coils are probably original if I had to guess. There is a very heavy gas smell coming from the starboard motor after running so fuel is not being burned somewhere and there are no signs of leaks anywhere around the engine or bilge or fuel lines leading up to and around the motor.

From what I’ve read this is a semi common problem. Any more insight on any specifics would be great.

Did this ever get fixed?? We have something close to this issue on a new Skater with 700's

Bold above should read Doesn't.

Just fixing a mispelling in case Griff doesn't notice and edit.

​​​​​​​Any engine in neutral requires little if anything of fuel to rev to a high rpm. Load is a whole another story, it needs tons !