350 mag MPI will not turn up past 3500rpm

[porscheguy]

So the engine is 2007ish, ecm 555, closed cooling. Under load/sea trial, or in neutral, the engine will not turn up past 3500rpm. Once it hits 3500 rpm, it acts almost like it’s hitting the rev limiter. It’s a very steady, rhythmic miss/cut out.

I have changed: cap, rotor, plugs, fuel filters. I have swapped the ECM and coil/ignition module with known good units. The problem remains. We borrowed a good crank sensor and swapped that on. We thought it gained 700rpm, but the boat tach isn’t always giving the best info, and my handheld tach had failed while diagnosing this boat. We did replace the crank sensor with a new one. And are using the original ecm and coil as swapping them changed nothing.

Overall, the boat runs excellent up to 3500. It starts easily, and jumps right on plane. We’ve plugged in diacom and can’t find anything amiss. We’ve got a lot of diagnostic time in this, and part of me feels the issue is a stupid one. So any input from experienced techs would be great.

 

[dubs283]

350 mag mpi bravo? alpha?

Correct prop is useful, more an art than a science

Other science would think a fuel supply issue may be the culprit

 

[alldodge]

From previous thread this is twin motors (for anyone looking) My mistake

Many auto parts places will loan a hand held Tach. If it backs up the boat Tach then its something else and scanner is needed.

With both doing the same it may be Rev limiter, knock sensor picking up detonation, sync if control has sync

 

[porscheguy]

Bravo 3 drive. Not sure on the props sizes. I should have mentioned we did run it on our day tank to eliminate the issue of fuel quality. And we did repeatedly get 3500 before my handheld tach failed.

We do have the diacom scanner. When it starts missing the spark advance isn’t dropping back. I thought if the knock sensor was getting triggered it pulls back timing?

SW pressure is 14 psi at 3500. No audible alarms.

I haven’t posted here in a few years. This is a single engine boat. No digital controls.

 

[alldodge]

I haven’t posted here in a few years. This is a single engine boat. No digital controls.

Sorry about that my bad will change my post above. Got mixed up and forgot where I was

 

[alldodge]

Do you have a in gear switch on the shift plate?
If so, unplug and try it

 

[QBhoy]

As far as I know, most or all mpi around that age have a rev limiter in neutral at a certain reduced rpm. Must admit though, I thought it was about 2500 rpm. Not 3500 rpm. If I’m wrong about that and it is 3500…I’d also agre with @alldodge and think around the neutral safety switch in the remotes…
But…if I’m right, and knowing what I know about my modern engines I have…I’d then think about the trim limit system or trim sender system. If yours is in anyway similar minded to one of my engines, if things are untoward there, it can limit the rpm available, if it thinks you are trimmed up beyond a certain point. Where it won’t allow you too much throttle for the sake of protection and mechanical sympathy.
Someone may correct me that knows better. But I know for certain that modern merc outboards have this function and resulting issue when they fail. My earlier 2003 mpi mercruiser doesn’t have the neutral rpm limiter like the 2005/06 onwards do, nor will it reduce rpms with too much trim into trailer…but your more recent mpi with the neutral limiter, might just also have the limiter trim/tilt protection limiter system too. Especially if you have the DTS control system, I’d also think. Could be wrong, but I like to try offer an out the box thought or two !

 

[porscheguy]

Do you have a in gear switch on the shift plate?
If so, unplug and try it

There’s some kind of switch on the shift plate. I presume some kind of neutral safety switch. I’ll unplug it and see what I find.

 

[porscheguy]

I unplugged the neutral safety switch on the bracket like you recommended and problem solved. Turned right up to redline. Thanks for the assist.

 

[Scott Danforth]

your motor didnt know it was in gear.

not reving in neutral is to save the motor from grenading.

so now fix the switch.

 

[alldodge]

Might need adjusting Or just leave it disconnected

 

[porscheguy]

New switch is on order. Customer boat. You know the drill. Safety gear has to be 100%. Plus the lady who owns it likes her alcohol.