Distributor/ignition issues, need help diagnosing
 

2000 Scarab, 350 mag mpi, s# 0l653009

Blew a pickup coil last weekend. I had a spare(old one I had) so I swapped it out after I finally figured out what it was. Drove Sunday with no issues. Went to take the boat out today and bam, same thing. Boat ran fine unloading, and through no wake area. About the time I went to take off the engine started missing real bad, and finally just gave out. Its not the coil I already switched them. This was the exact same thing that happened last weekend. Is there any thing that could be causing these to go out? Short maybe? The pickup coil on the good engine reads ~10 ohm between leads. The bad one reads ~3, as does the one I took out last weekend (I still had it laying in my shop so I used it for reference). I know I can just change it out again, but I don't want to be changing them out every weekend. Any advice is welcome. A troubleshooting flow chart would be great too. Only one I found was one for the thunderbolt ignition, which is different.

wow, 251 views and not one response.

Can't help to much with a trouble shooting diagram, but there isn't much to the system other than power, the coil, and the ignition control module. See about ordering a Mercruiser manual, I'm sure it would have troubleshooting steps with voltage reads at the various locations.

Is this diagram your system?
http://www.mercruiserparts.com/Show_...sp?dnbr=816596 98&ivar=images/COMMON/14007.png&inbr=2697&bnbr=60&bdesc=Distibutor+And+I gnition+Components
http://www.mercruiserparts.com/Show_...sp?dnbr=816596 98&ivar=IMAGES/COMMON/14006.png&inbr=2697&bnbr=50&bdesc=Wiring+Harness+A nd+Electrical+Components

no picture showing up when I click on the link.

let's try this. You might have to copy and paste into the browser.
http://www.mercruiserparts.com/Show_...sp?dnbr=816596 98&ivar=IMAGES/COMMON/14006.png&inbr=2697&bnbr=50&bdesc=Wiring+Harness+A nd+Electrical+Components

http://www.mercruiserparts.com/Show_...sp?dnbr=816596 98&ivar=images/COMMON/14007.png&inbr=2697&bnbr=60&bdesc=Distibutor+And+I gnition+Components

Didn't work so just go here and select the right picture.
http://www.mercruiserparts.com/selec..._nbr=816596+98

If your ignition system seems to check out fine, I would check into the fuel delivery, for that motor. Fuel pump, filter, filter on the carb, or the carb itself could be ready for a rebuild. If the accel pump is gone, the boat will start and idle fine even rev in neutral but once placed under load it it will sound like it is missing hard, maybe pop thru the carb and die.

Its MPI not carb. I did think of this but I replaced the pump and fuel filter middle of last year. It is definitely an ignition issue. When it starts to go out, it will run on plane but is missing bad, I can feel it not hitting on a cylinder and then hitting hard on the next. If its a fuel issue it would not act like this. I called Mercruiser and talked with a Tech. He stated that the system is really quite simple. One wire comes straight from the computer supplying 12v which I tested and have, and the other goes to the injectors, with the third going to ground post. He said the only reasons they go out were because of age, heat, or arcing within the distributor. He suggested changing out the cap and rotor. I changed those out last night, along with a new pickup. I did read somewhere to use di-electric grease on the bottom of the pickup to help with heat transfer, which the Merc tech agreed with. I'll post back when I lake test it.

I would make sure all the grounds are making good contact.

NO NOT DIELECTRIC GREASE! You need to use heat paste!!! I made this mistake and fried a new pickup.

Shoot I missed in your op where you said it was mpi.

So is the issue with one specific cylinder, or does it seem to move around? Are you having any other symptoms, hard starts, rough idle, any signs of break up, or popping.

What brand of hei is it?