496 Hydrolock

Tomco

Active Member
Aug 16, 2012
455
Ottawa, Ontario
Boat Info
2004 420 Sedan Bridge
GHS Hydraulic Lift
11' Walker Bay, 30hp
Engines
Cummins 6CTA 8.3M
I have the 496 (closed cooling) with Bravo III's - last week when out the starboard engine hydrolocked. We had idled into a lock and the engine would not start when exiting ... it sounded a lot like a dead battery and as a result of persistent attempts I spun out the starter. Back at the marina the mechanic installed a new starter and on first attempt it locked up again and he recognized it was hydro locked. Turned out there was a little water in only one cylinder. Pulled the offending plug and turned over to blow out the water ... ran fine afterwards. Headed out today and we had the same scenario play out again! I recognized the situation this time and pulled the plug (same cylinder), turned it over to blow out the water and it started right up and lasted fine for the rest of the day (at least 7 starts/stops).

In both cases, we were on plane for about a half hour or so before slowing down to enter the locks - not sure if that is relevant but we only went at displacement speed afterwards and had no problems.

I am thinking that it is most likely the gasket between the manifold and the riser leaking ... would appreciate any any insight from anyone who may have had a similar problem.
 
Yes, the riser/manifold attach point is the first place to look. Please do this before attempting to start the engine again. You're very lucky.

Good luck!
 
It is know time to break out 4 boat dollars and change exuast manifolds just for parts labor is 2 or 3 more.
 
The riser/manifold on these engines is a "dry joint". No water passes through that spot. Raw cooling water enters the bottom of the exhaust manifold and is pushed upwards to the top of the manifold where there are two small hoses that bypass the riser/manifold connection. Raw water continues up through the two hoses to fittings on the side of the riser, then out the end of the riser where the rubber exhaust hose starts.

My guess is that your exhaust manifold has developed a leak inside the inner wall. That's allowing raw water to leak into the dry exhaust area upon shut down. That leaking water is finding its way to the nearest exhaust port and getting into the cylinder.

The only thing that may have saved your engine here is that by design, the engine management system will not provide spark to the plugs until the engine has gone one revolution (from what I've read anyway). Without that, it would only take one of the other cylinders to spark and "poof" time for a new engine!

Remove and inspect the manifolds...
 
Not sure why it would be different on a big block, but a closed cooled small block has antifreeze pumped into the bottom of the manifolds and exits the spacer between riser and manifolds. Riser is cooled with raw water.

Is water or antifreeze getting in? Pull the risers and have a look. Those manifolds should last a long time.
 
Not sure why it would be different on a big block, but a closed cooled small block has antifreeze pumped into the bottom of the manifolds and exits the spacer between riser and manifolds. Riser is cooled with raw water.

.

Different, all Merc 8.1L/496cid exhaust parts are raw water cooled.

I'm not familiar with a 340 with outdrives but, if you have a Y-pipe after the exhaust elbows that carries the exhaust gases and cooling water out...there should be shutter valves in there to prevent back flow.

Just a thought,

Good luck,
 
I have the exact same boat as you and I just had the same issue this spring. I had the boat Put in the water, started on the trailer and ran fine to the slip where I get it ready for the run to my home port. Shut it down for 1hr and it was hydro locked upon started up. I fried the starter same as you thinking it was a dead battery. I realized it was hydro locked at that point so I pulled the plugs to verify. I just did all new manifolds last year so I don't beleeive that is my issue but I wanted to make sure so I changed out the manifold on the exhaust bank that I had the water. I have not had a problem since with about 30hrs on it with probably 30 start/stops. I am still wondering what my problem was. I pressure tested the old
Manifold and it held pressure so I don't think that was the problem. The only thing I can think of is that when I was putting the radar back on the arch I hit the start switch with my foot for a second. The motor didn't start but I wonder if it ran backwards for a revolution and sucked water in that way.
I know this isn't much help to you but I am interested in your situation because we have the same boat. Hopefully you are all set know but post anything if you find out the problem.

Good luck.
 
Thanks for the comments.

I'm planning on pulling the manifolds and risers for inspection ... I think Jimmy has probably diagnosed it. I did check with the mechanic and it turns out it was a different cylinder (#7) vs. (#3) when it happened a couple days ago. I plan to pull the manifolds and risers for inspection - is a pressure test the only way to confirm if the manifold is leaking?
 
Got some time with the mechanic today ... nothing definitive could be found!

So here is what was done:
1. compression test - 125 - 135 psi
2. check of the flapper valves - it was not there! so we took the other side off and check, it was not there either so we think they were never installed.
3. riser removed and checked for leaks - they did this by filling the water side with gasoline and looking for seepage into the exhaust side. No leak was found.
4. manifold removed and pressure tested to 20psi - it held the pressure.

Given that the water ingestion doesn't occur every time, I didn't think a definitive problem would be found. They have two schools for thoughts for continuing 1. replace the manifold (most likely cause) and try it, or 2. pull the heads and look for a problem on the fuel side that may be causing it to create a vacuum and pull water in.

I'm inclined to go with changing the manifold and giving it a try. Not sure if I should also replace the riser at the same time ...

Anyone else go through this and have comments?
 
Got some time with the mechanic today ... nothing definitive could be found!

So here is what was done:
1. compression test - 125 - 135 psi
2. check of the flapper valves - it was not there! so we took the other side off and check, it was not there either so we think they were never installed.
3. riser removed and checked for leaks - they did this by filling the water side with gasoline and looking for seepage into the exhaust side. No leak was found.
4. manifold removed and pressure tested to 20psi - it held the pressure.

Given that the water ingestion doesn't occur every time, I didn't think a definitive problem would be found. They have two schools for thoughts for continuing 1. replace the manifold (most likely cause) and try it, or 2. pull the heads and look for a problem on the fuel side that may be causing it to create a vacuum and pull water in.

I'm inclined to go with changing the manifold and giving it a try. Not sure if I should also replace the riser at the same time ...

Anyone else go through this and have comments?
 
I think the missing exhaust flappers could be the culprit i would replace those ASAP - on a 12yr old boat it's hard to say if they were never installed or fell out. No obvious leaks found in the manifold, but yet it hydrolocked that suddenly. A leak in the manifold large enough to fill a cylinder with water in the time it takes to go through a lock would certainly show up in the testing you have done.
 
Update: I spoke to Mercury late this afternoon and walked through the issue with them. As BillK pointed out, they too recommended that replacing the flappers was the first order of business and that could very well be the problem. Mercury also recommended running the diagnose for fuel and electrical (software?) to investigate/rule out anything that may cause the engine to run miss or run back at shut down.

Tracking down the flappers now ... will try that first before changing anything else.
 
Are the manifolds aluminum or cast iron? Our vintage 8.1's in 340 Sundancers had aluminum manifolds and if your boat had any salt water exposure they should be replaced with cast iron. Seeing your location maybe it has been in fresh water all it's life it would probably be okay with aluminum but if the boat was in salt water it will diminish the integrity the aluminum. Mercruiser went back to cast iron after a few years of failing aluminum manifolds in salt water.

I'm guessing a "flapper" is an I/O thing at the transom end of the exhaust exit?
 
Update: I spoke to Mercury late this afternoon and walked through the issue with them. As BillK pointed out, they too recommended that replacing the flappers was the first order of business and that could very well be the problem. Mercury also recommended running the diagnose for fuel and electrical (software?) to investigate/rule out anything that may cause the engine to run miss or run back at shut down.


Tracking down the flappers now ... will try that first before changing anything else.

When I took mine apart I did not have flapper either. Neither of my engines did. I have put over 300 hrs on my boat and this is the only incident. It is interesting that Mercruiser says to put flappers in now. I am going to investigate that further.
 
The only thing that may have saved your engine here is that by design, the engine management system will not provide spark to the plugs until the engine has gone one revolution (from what I've read anyway). Without that, it would only take one of the other cylinders to spark and "poof" time for a new engine!

Remove and inspect the manifolds...

Jim, when I spoke to Mercury I remembered your comment and asked if this is indeed a design feature - they confirmed that indeed, the engines are designed this way. Got to give that engineer credit ... probably saved my engined!
 
Are the manifolds aluminum or cast iron? Our vintage 8.1's in 340 Sundancers had aluminum manifolds and if your boat had any salt water exposure they should be replaced with cast iron. Seeing your location maybe it has been in fresh water all it's life it would probably be okay with aluminum but if the boat was in salt water it will diminish the integrity the aluminum. Mercruiser went back to cast iron after a few years of failing aluminum manifolds in salt water.

I'm guessing a "flapper" is an I/O thing at the transom end of the exhaust exit?

The manifolds are aluminum. The boat has only floated in fresh water ... looking inside the ports and they look like new. No corrosion at all!

Correct, the exhaust shutters are in the "Y". Mine are right at the water line.
 
When I took mine apart I did not have flapper either. Neither of my engines did. I have put over 300 hrs on my boat and this is the only incident. It is interesting that Mercruiser says to put flappers in now. I am going to investigate that further.

The service tech I spoke to made the point with conviction - he felt strongly that the shutters should be there. At one point my marina had called the local MerCruiser dealer to clarify if the shutters are suppose to be installed (they don't show up on any diagrams), they called Mercury who confirmed that they should in there. I'm assuming that the phone number I called was different than the dealer so we likely spoke to different techs.

If you call, post what you find out.

I'll be installing them early this week and will then do some extensive water testing to see if I can re-create the hydrolock problem. Will post the results.
 
Bill, just curious about these occurrences and Ottawa's locks. Is this happening only when using the locks? Is there any possibility that the water turbulence while waiting in the locks is strong enough to actually force water into the exhaust ports on your outdrive? Those missing flappers are meant to stop water from entering your system from the reverse exhaust direction.

It's a stretch, but just trying to think outside the box...
 
Jim, I assume you mean Tom (not Bill)...

In one case we were going up a lock (water rushing in) and going down (water heading out) in the other case. It is an interesting thought. As part of the water test with the new shutters I will be going through a couple locks.

Cheers,
 
An update in case anyone else has this problem and follows the thread.

Installed the missing exhaust shutters and that seems to have fixed the problem. Ran the boat on several higher speed runs, pulled the plugs on the offending side and they were dry and burning nicely. We went on a 4 day trip, with several sections on plane and then at idle speed into the locks - no problem. It is not an expensive part (less than $100 for two) and easy to install.
 

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