'86 300 Weekender - Randon engine shut downs

[joenofish]

We have an '86 300 Weekender with twin 260 Mercruisers, both engine start and run perfect...at first...
at random intervals, the starboard engine will shut down and not restart, it will eventually restart but I never know when, could be twenty min or three hours, then runs great for an unknown amount of time, 30 min or two hours.

I replaced the coil and pickup in the distributor no change.

Any ideas?

Thanks,

Joe in St Pete

 

[1956olds]

sounds like a fuel pump going ,are they mechanical or electric? The other thing may be the thunderbolt module.

 

[Alegria]

Had a very similar issue last year. Oddly enough found that the starboard alternator was bad causing the battery to drain and the port alternator could not run both engines. Just thought I would share. The engine would start with after the port Alt put some charge into the battery then would shut down. I left the charger on the battery during the week and made it tough to diagnose.

Sent from my Galaxy S3

 

[Saverio]

I had a 30 express with same issue. It kept wanting to die than would run fine. I put a meter on coil and had 9volts. I started at key switch and worked back. the screws on key switch was loose. once tightend it was fine

 

[Tritone]

I had a similar problem on my stb. motor. After a lot of searching, it turned out to be the fuel pick up.

 

[JVM225]

It could be any of the things already mentioned. You need to first find out if it is a fuel or a ignition problem. The best time to figure that out is when it won't start back up for you. See if the carb is getting fuel, if not work back from there.
If the carb is getting fuel, then you need to see where in the ignition system the problem is.
Easiest thing to start with is to swap the Ignition modules to eliminate that possibility.
I had the exact same problem 6 or 7 years ago with my stbd motor. Changed the pick up and the coil and still had the problem. It turned out to be the Thunderbolt Ignition module. Finally diagnosed it by swapping the modules on the two motors, and sure enough the problem went to the port motor. Replaced the bad module, and problem disappeared and stayed gone for the rest of the time I had those motors in the boat.

 

[PMC]

It could be any of the things already mentioned. You need to first find out if it is a fuel or a ignition problem. The best time to figure that out is when it won't start back up for you. See if the carb is getting fuel, if not work back from there.
If the carb is getting fuel, then you need to see where in the ignition system the problem is.
Easiest thing to start with is to swap the Ignition modules to eliminate that possibility.
I had the exact same problem 6 or 7 years ago with my stbd motor. Changed the pick up and the coil and still had the problem. It turned out to be the Thunderbolt Ignition module. Finally diagnosed it by swapping the modules on the two motors, and sure enough the problem went to the port motor. Replaced the bad module, and problem disappeared and stayed gone for the rest of the time I had those motors in the boat.

Agreed, spark or fuel. When it fails to start, pull a plug wire and see if you have spark (ground a spare plug, don't touch the end - LOL)