overspeed kills and im dying

i have a 8.1 496ho and its killing me. when i run my boat everything is good till i hit 4200rpms. i have already changed the coolant sensor, the exhaust manifold sensors,the whole impellar housing and no change. once i hit over 4200rpms the horn blasts solid and the boat shuts down to an idle. i went out yesterday with the rinda scanner and every time i broke 4200 it would happen, when i check the scanner two codes come up. maximum rpm overspeed #19 and gaurdian strat limited power #9. up to 4200rpms everything is ok. help me anybody please

Is this a new problem?
IFSO... what changed (if anything) before problem?

how are you thankyou for responding. as for my problem it just started out of nowhere. a friend of mine said it was probably a water issue and told me what parts to change but when nothing changed i decided not to chase things without knowing so i went out and bought the rinda scanner. i know the codes that come up and i know im not overspeeding for that to come up. everything runs good i can run all day at anything up to 4250 rpms

sorry sandkicker this procees is all new to me so im not sure if you are even getting this post

They way this process works is that anyone who logs onto this website and forum sees everything posted by everyone. Soooo... at anytime any number of folks ( most of whom know more than I as some are top notch professional marine mechanics) will chime in on a topic. My SWAG is that since it came on suddenly and is at a specific speed is that the ignition module which contains the rev limiter circuit has an issue. Other wiser heads may have other thoughts.
I'm not familiar with your particular engine, but what is the max rpm range marked on the decal on the flame arrestor?

the max rpm is 5000 and i have never broke 4900rpms and the motor only has 142hrs and i do maintain it. i have also changed the iac valve on it to.

im sorry it is a 8.1 liter 496ho and the serial num is 1a042122

Service manual 33 is what I think you need.......Maybe someone can give you a link for it.

Pm me (personal message) with you email and I can try to send you the PDF manual. it is ~ 7 MB in size.

From what I read, and I do NOT have any experience with EFI so.........

The code 19 does not seem to refer to any sensor failure.it refers to things like a spun hub in the prop, over rev in neutral, cavatation, thus allowing over rev condition thus code 9 (guardian mode).

That being said. You say it will not go above 4200 before this happens.........

Have you reset the codes?

Maybe disconect the battery for a minute or two and reconnect. Let everything reset.

One issue I have seen reported here is bad contacts on the engine harness connectors....Pull them apart and look close for corroded contacts (pins and sockets). A weak signal froma weak contact could cause a wrong reading with respect to you rpms......

Maybe use a small amount of dielctric grease on the connectors and reseat all of them.....

There are only a couple of guys here that have any real EFI experience.........Maybe they will chime in and give you a better answer......

FROM MANUAL;
One thing in the manual says:

Continuous horn and 5-90% available power:

Battery voltage,
Guardian strategy,
over speed,
ECT over heat,
ECM memory,
Multiple CAN circuit faults,
EMCT over heat

This one looks interesting..
DTS SENSOR FAILURE..................5% Available power

Last one is :
Engine oil pressure......Forced idle

the hub is solid and i just picked up the manual which is informative but i dont see anything that it could be. im gonna check the pins and sockets and will pull the cables off the battery for a couple of minutes to see what happens and hope for the best

also i dont have dts and im not familar with can fault circuits

Check the shift event switch. If you are getting an overspeed alarm, the pcm thinks the drive or transmission is in neutral. You can check this with a scan tool

i had the rinda scan tool on it when the alarm went off and i reset tested it several times. when i looked at the history faults it said it was in gear and also that the code is inactive whatever that means

any chance you have replaced the alt.?

dupe posting

no everything seems to be charging fine.could that be the problem?

came across this posting about the same problem, it seems the overspeed signal may come off the alt to the smart gauge system.
http://www.boatbanter.com/showthread.php?t=74782

you could disable the alt and do a test run or put a good voltmeter at the battery and record the actual voltage at that rpm

i will have to look into that. i was also told that maybe the transom tube could be clogged and when i hit 4250rpms it might not be getting enough water

Don't these engines get their rpm from a sensor?
I know an automobile reference may not be applicable but if this happened on one of my cars I'd be looking for whichever sensor sent the computer the engine rpm signal.

Hi Did you solve the issue? I have a mercruiser 5.0 mpi, everything works well below 4200 rpm, above 4200 rpm the overspeed alarm went on. thanks Martin