4.3 merc post overheat

1995 Wellcraft with 4.3 Mercruiser V6

First day putting the boat back in the lake after de-winterizing and spring clean. I was moving slowly out of a very long no wake zone and one of my alarms went off. I turned around and noticed a little smoke coming from the engine compartment. I opened the engine compartment cover to discover a ton of water in the engine bay. Immediate response was that I had blown one of the hoses I had just put back on, but I looked at the temp gauge and it wasn’t reading anything out of the low range. I remember confirming the drain plug before I out the boat into the water. What could it be? Blown Bellows (of which I really didn’t know much about other than hearing your boat could sink if you have a cracked bellows) I re-checked the temp gauge and it was reading low. At this point my concern was taking on too much water. I tried to get to shore or a close by dock with a lift, but I was taking on a lot of water. I cut the motor and monitored the bilge and water depth. (Which was up to covering the oil pan and starting to fill the compartments by the batteries and storage.) I decided that I needed to get close to shore. I called my friend that was taking my trailer back to storage and asked them to return to the ramp. I started it back up (starter really struggled) and headed towards shore. After a few minutes the temp gauge did show a 240+ reading. Again smoke was coming off of the rubber exhaust boots and I felt like the motor was stressed, so I cut it off and was able to wave someone down to tow me in.

I walked it onto the trailer, headed home and parked it in the driveway for later review after cooling.

Following weekend…
Net, the main intake hose had come off at the motor so I was just pulling water into the boat and not into the motor.
The boat would not crank over. I thought I had seized it. I put a wench to the front pulley and it moved easily. I then pulled the starter and discovered that it was completely full of water.
Starter teardown / rebuild / reinstall and it cranked perfectly.
Pulled plugs and they looked good.
I ran compression on all 6 cylinders cold and got 143-150 in each.
I decided to do a warm up with the stern drive water ears.
Ran at low RPM for 5-7 minutes.
Checked oil for water – none.
Check exhaust water for oil – none.
Pulled plugs, they looked great. Re ran compression 149-151 on each.
At this point I’m hoping that I did not do any major damage to the motor, heads, etc.
I need to get the boat in the lake for load testing.

Now to the exhaust.
All 4 rubber boots on the exhaust had High heat damage.
I pulled the boots and the elbows to discover that the shutters (new 2 flap design) had melted to charred rubber.
After reviewing how the shutters were designed I discovered that there would have been a piece of metal in each shutter. I did not see them down in the Y pipe. I raised the stern and was able to tell that the metal pieces were in the exhaust bellow.

I pulled the stern drive to remove the metal.

When I was able to crack the stern gasket and pull the stern water started coming out. My thought was that this was coming from the water in the intake hose to the motor. After removing the stern I noticed that the bellows had water in it. Not a whole lot and it was very clean water. The grease was good. The ujoints moved perfectly and there was only a little surface rust on the one of the u joints. I looked at the bellows and it is in perfect shape.
I was successfully able to pull all of the remains of the shutters, no other damaged discovered.

Now to my questions (sorry for the long read)

Is it possible that the bellows filled up with water from the water that was in my bilge?
(It did get high enough to cover the starter and oil pan).

Or

Is there any other way for that much water to get into the bellows without the bellows having a crack.

I really don’t want to take it in to a shop to have a full bellows job until I can get the boat back in the water for further testing to determine if I did and major damage to the engine.

My plan is to reinstall the stern, install new exhaust shutters and rubbers and Lake Test.
Any other recommendations?

Again sorry for the long read and thanks in advance for any help.

 

The presence of SOME water in the bellows is normal. There is no perfect seal between the cavity and the engine bay. With water in the bilge, humidity is 100%, so condensation takes place on the coldest surfaces like the gimbal housing. That also explains why the water is clean.

 

It is worth taking the drive apart and look at the water pump upper seal, shaft upper seal and shifter seal. They are designed to work on a wet exhaust system. With the high temperature of a dry exhaust they are most likely damaged.