Merc 4.3 water leaking from top of block

[Boating Enthusiast]

So we experienced a strange issue this past weekend. Boat ran fine out of the marina and up the river to our favorite swimming hole. On returning to the marina, the starboard throttle was seriously out of sync (RPM-wise) with the port engine. 4200 or port while 3500 max on starboard and starboard throttle pegged. We dropped back off plane and at 1500 rpm both engines and throttle positions in sync. Tried to get back on plane and could not.

After arriving back home and getting on the lift, I noticed a lot of water seemed to have gotten sprayed all over the top of the starboard engine. Lowering the boat back into the water, I noticed water welling up through a pipe plug on top of the block just to the right of the thermostat housing. Appeared to be a plug where I was told an auto would have a heater core hose attached. The plug had a recessed square for using a tool to install/remove (bigger than 1/4" drive and smaller than a 3/8 drive so couldn't find a tool to remove easily. Stuck a big flat blade screwdriver into the corners and that worked to remove).

Due to the thin wall of the bottom of the square recess in the plug, over time this seems to have rusted and developed a crack/hole. Pic attached. This was probably spraying water when running the engine and might have been getting some water sucked into the air intake causing air/fuel mix to be off and preventing the RPMs to sync with the port engine.

I pulled the pugs, cycled the engine to push any left over water out of cylinders, fogged, and replaced plugs. I found a brass pipe plug with a square head (instead of recessed) and re-installed in the top of the block where we removed the failed plug. Seems to be running at idle fine. Ran out of time to take out for a full run and check at higher RPM/load.

My questions to the group:
Why would this have failed? Stored on lift so not thinking electrolysis and anodes have always appeared fine.

Should I replace the platinum tip plugs (looked ok and seemed to idle fine) and if so should I replace both engines?

Should I use Red RTV sealer on the block plug? Appears to have had a coating both on threads and bottom of the previous plug. Any concerns with this getting into the cooling system or block passages?

Should I replace the other engine top plug?

Thanks for you help.

 

[scoflaw]

Rust never sleeps. Flush all you want and the rust will continue, nature of the beast.

 

[Bt Doctur]

if it was a cast iron plug, simply rusted away
use some type of sealer and just snug ,its a tapered thread no need to go crazy tight

 

[NorCal Boater]

Looks to me like someone used the wrong plug and it just failed. Use the brass plug, a bit of sealer and only change the plugs (spark) if they're due. Just one of those things....

 

[skibum]

With all of that water spraying around, I wouldn't be surprised if you just had wet wires causing arcing. You might be fine after everything dries out. I'd just wait until you can run it before doing anything else.

 

[KevinC]

I had a similar issue happen on my 260DA with the 5.0MPI. One of those plugs rusted and developed a pinhole leak that was spraying water all over the engine room. It was a very simple repair - my mechanic pulled the plug out and replaced it with a much thicker brass plug. I was surprised at how small the plug was that came from the factory in comparison to what it was replaced with. As was said before rust never sleeps and it took it's toll on my plug too!

-Kevin