496 mag shuts off going down the lake
 

I have a 496 Mag 2000 year, from time to time it will just quit while
going down the lake. The gauges go flat and nothing. Turn the key off and restart it runs fine. I have been over everything, grounds
fuse links and nothing seems to be out of place. I have noticed the
ecm on the engine has been replaced in 2003. I bought this boat in 2009 and have been plagued with this from the beginning. Any thoughts or where to start. I find myself to be good at troubleshooting but this one has me baffled. Thought about taking it to a dealer but I think I may know more than they do in my area and eliminator is a long way to drive.

Thanks in advance for the help.

vapor lock?

not if the gages go flat... thats massive electrical disconnect... main bat lead/ bat switch/ not sure about lanyard switch... does that just kill the ignition or power to the dash... and by gages going flat, you are saying, " as if you turn the key off" correct ?

and the fact that you can reset it makes it sound like something shorting to ground on a " bump" . i had a car do this and it turned out the battery was loose and bouncing up and grounding on the hood. i only found it after it finally stuck and burned a hole in the hood...

496 Mag
 

I would agree that it could be except the gauge issue when it
shuts off they go flat and it does not lead into any pre signs of fuel shudder just running one second and off the next like you turned the key off. I bypassed the lanyard switch thinking this may me the issue but no luck.

Thanks for the help.

i just thought of something... what if that giant plug that the harness plugs into the motor at the rear is corroded or loose... that would do it for sure

Ya it could be but it is clean and no corrosion and you can almost pull it completely off before it would kill the motor. I had a buddy with the same set up have the same problem. His fix was to install a whole new wiring harness which cost him 4K. Was hoping no to go there. Thanks for the help! I will keep looking.

Possible Solution?
 

I believe the problem is the Crank Position Sensor which is found on the back of the motor.

If you are standing in front of the motor it would be on the right side rear of the block.

This is a known issue on the 8.1 of that year.

Good Luck

Is there a way to test it? ohms ect? Would this make the gauges go flat also?

I sure will look at it. thanks for the help!!!

could be a relay,,,

Usually a bad crank position sensor there is no tach working ( I have never seen the tach working with a bad crank p.s.) and alot of times no restart. The other gauges will work with a bad crank p.s. You have a stock 8.1 L / 496 mag 375 hp Mercruiser correct because the 496 was introduced for model year in 2001. Just asking.

Also have it scanned for codes and as soon it shuts down does any other power work like engine hatch, blower, lights and so on. Also as it shuts down ASAP check your voltage at all points meaning start at the battery cables, to the perko switch, to the starter, and so on also check that 90 amp fuse at the starter for voltage & connection. Other things to look at connections to the ecm, key switch connections and kill lanyard. Hope this helps.