496 HO 2004 - Shutting down when throttling down
 

I am having this problem for the third season. (posted this issue last year).

My motor shuts down occasionally when throttling down (skier falls), coming into shore, etc. Once the motor shuts off it typically will not restart for 25min-2hrs there has been a few times where it would restart immediately (rare). Last season I changed the IAC, and mechanic ran scanner changed #5 plug, ran stronger but died same day again. This happens almost every day I am running, worst case 2 of 3 days. It is not throwing any other codes, fuel pressure on rail was showing between 40-44 psi consistent. Pulled all interior and followed all wires to look for cracks, chafing etc., nothing, unplugged and reconnected most sensors and plugs.

From other posts I see that it may be fuel, or Crank Position Sensor however it does not sputter shuts down clean, seems to be electrical. The motor cranks similar to when the kill switch is tripped but will not start for the 25min-2hrs . Trip next weekend, trying to get a piece of mind before then, any help is greatly appreciated.

Thanks!

Crank position sensor should throw code, do you have spark , fuel when it won't start?

There are no codes other than from what the mechanic says, codes that always show. I did not check for spark when this happened, I know rookie move. If it happens again, which it will I will pull each wire one at a time and crank motor see of there is spark. I am 99% sure that I can hear the fuel pump priming when I try to restart. The last time it happened things were hectic, I was floating fast into some rocks, so I could be wrong trying to do stuff real quick.

Aalmost need to barrow or buy a scanner. This way you can see what the computer is seeing when this happens.

but back to basics. Engine needs air, fuel, and spark to run. Take away one and it wont start. My guess is your loosing spark, and need to test or it next time it quites. Do you have any way to keep a fuel pressure gauge on your fuel rail wile running?

Crank position sensor if it is bad you will not have a tach signal, So is your tach always working ?

IMO bad IAC again, or shift switch bad or out of adjustment or a problem with the transom harness. Earlier 496's did not have a shift switch as it was determined by serial # not model year. I am almost 100% that all 2004 - 496's had the shift switch incorporated by then. I would not rule that out as being your problem. It is a huge lunch eater for many marine techs who work on this stuff professionally.

Do you have smartcraft ? What is your fuel pressure as the motor shuts down on its own and when you are trying to restart the engine after that. More info is needed. Also make sure your connections to the cam & crank position sensors are well connected and no corrosion. Merc has had some problems with these connections and the transom harness connections.

Bad kill switch?

Like Sydwayz said check your kill switch. Was thinking about this today, it almost sounds like you are losing power/ ground when engine is hot. Pull your fuel pump relay (right side top of manifold) and look for any melted plastic or discoloration on the terminals. Some Mercury black motors had the female pins on the harness lose there tension when heated multiple times. This would do what you discribed and will not through a code.

This might help as well, so click on the link below pretty close to your problem. As I believe your problem is in the transom harness and or 5 volt sensor circuit in which all tie in. If not do not rule out the shift switch out of adjustment or bad. I mentioned all of this in post 5. Have your marine tech check out these areas, hopefully he is well trained on all the PCM 555diagnostics procedures.

http://www.offshoreonly.com/forums/d...y-warning.html

It just came to me - you posted this last year as well and I answered about the shift switch and I believe Mercruiser support told you the exact same as I remember now. I have a really good memory plus Brain gave you the same answer as well last year. Kill switch. Your problem is in post 5 or 8. I'm sure you had the easy stuff looked at already correct and your problem is still taking place.

I have some more information on my engine/ setup after phone call with Mercury support this morning.
496ho built 3/25/2004/ Sterndrive / Bravo I drive - Cool Fuel Gen II - Two fuel pumps: Boost pump and High Pressure. I asked Merc about the "Shift Switch" they said this motor setup does not have this, that a shift switch is related to an I/O with transmission (or something like that). They did advise me to pull the Crank Position Sensor and look for wear/ corrosion etc.. Advised to pull the connection to the fuel pumps to look for signs of melting/arching etc.

I will be changing the Fuel Water separator and the inline fuel filter in a couple of hours, I will then test fuel pressure and pull pick up tubes in the tank to check for gunk/ cracking etc. It does not appear that the kill switch is bad, as when I flip it the motor dies, and flip it back it will fire up. Should I change it out as a precaution? I will also follow the wires back to motor to look for any problems with wires/ connections with this switch.

Thank you to everyone for taking the time to assist me with this issue