496 Mag - Blown motor at 9 hours

[kub4330]

Sn# 1A338275

Boat (268 bryant) has 9 hours and has a blown motor supposedly from hydro lock. The Senario: I was coming into the dock after a 20 minute run where the boat rpm was varied from 2800 +/- to around 3800 - 4000 rpm. Slowed down through break wall and idled at 1000 rpm in a no wake zone to the dock. Along the way I shut the motor to talk to a friend and when I went to restart it was very sluggish turning over. When the motor caught it was followed by a loud knocking sound. It has been determined at this point to be a bent connecting rod as everything on the top end was o.k. The cylinder with the problem is # 7. Compression was 140 vs 175 in all other cylinders.

Merc is going to replace the block, but why this happened in the first place is more of a concern to me right now. I want to avoid this in the future. I have read a lot about the issue of hydrolock and don't think the wake against the back of the boat should/would have caused this as it was not an abrupt. Flappers? Manifolds? Engine timing?

When the new block comes the intake, manifolds, fuel system, pulley driven accessories will be swapped out. The mechanic is highly regarded, but I want to make sure I quiz him in the areas of concern. I don't want to be a pain in the ***, it was not his fault, but I am not the type to sit idle when there should be a reason for the issue.

Advice?

 

[HT32BSX115]

Re: 496 Mag - Blown motor at 9 hours

Howdy,


A hydrolock caused by water in a cyl is usually not able to bend a rod when you crank it.

Starter motors usually don't produce enough torque to break anything......I suppose it's possible with one of the 'high-torque' geared starters................... but it usually just stops the cranking.(I had a Ford OMC 460 do it every time I shut down and restarted for a couple of weeks)

If you get water into the intake somehow, when the engine is running it will be a whole different story.

The rotational momentum of the crankshaft and rotating mass is more than enough to break something when there's a sudden stoppage of a running engine.

You didn't indicate the year model and I didn't look up the model number.

If you have "Dry-joint" manifolds and risers, your water intrusion did NOT come from a leaking riser gasket etc There is no easy path (riser gasket) for manifold/riser cooling water back into the exhaust area.........It's the whole point of "Dry-Joint" manifolds and risers!

If it DID come from a leaking (cracked) head, intake manifold gasket etc, then it was not your fault and there was nothing you could do about it.

If you had a "bad" or missing exhaust shutter water could have rolled up over over the "hump" (riser) if you have a big rolling wave from behind.

If you shut the engine down while underway it could absolutely happen. DO NOT shut the engine down while you're underway ay any speed unless it's an emergency. AND if you do, understand that you could hydrolock the engine.


Since you're under some sort of warranty and Mercruiser is replacing it, they probably think you didn't do anything wrong either.

Cheers,


Rick

 

[Tail_Gunner]

Re: 496 Mag - Blown motor at 9 hours

Cant be certain but there is a condition called reversion. Made short water is capable of going upstream back into your block thru the exhaust system. This is caused by more than a few condition's..one being your motor is mounted to low in the boat and another is using a cam that has a lot of overlap acutally sucking water back into the block..Iam sure you at some point have heard a high performance car idle with a very rough idle a extreme example but that lump you here is the intake and exhaust valve both open at the same time..It creates more power at higher rpm level's

Slowed down through break wall and idled at 1000 rpm in a no wake zone to the dock

That was probably when it occured...your boat is sitting at it's lowest point and the cam reversion pulse's were at there peak..little revesion comes after 1500 rpm at that point the positive exhaust flow negates the reversion pules.

So a few things..Static Line and cam size and since that was a factory merc i would look at Static line close..here's a example and right here on iboat's http://www.marine-engines.net/topics/001078.html?motors..>A bit more might make it clear...That's a big block engine not only does it have a large bore but also a long stroke so when it is on its intake stroke it really pull's and your on a larger boat (squatting) deep draft which allow's the water to back up into the exhaust system.

http://www.marine-engines.net/topics/001078.html?motors


Static line The way to determine true static line is to get a 1 inch clear hose about 12 feet long. Put one end over the boat and in the water. then take the other end put some water in it. Take the end that is not in the water make a loop in it with water in the loop. Lower the loop below the engine and keep the end above the engine. So you have a loop with water in it and the end above the engine the level in the hose is the true water line. You make a mark on the engine usally on the exhaust manifold then measure up to the top of the riser this distance is the static line. Static line for a Flat tappet cam shafts was 8 and 3/4 inches. The new roller cam requires 13 and 3/4 inches of static line so that the engine will not ingest water in thru the exhaust at idle. You can take and engine and run it out of a boat with out water to it and put your hand over the exhaust at the riser and feel the positive pull on the exhaust at idle. If there is water present within that 13 and 3/4 it will be pulled into the engine thru the exhaust side. We have made an example to show customers an engine with clear exhaust hose and put water in the hose and you can watch the water being pulled into the engine. If you are milking up the valve covers the water is being pulled in thru the exhaust up the exhaust valve and in to the valve cover. static line if the boats seen the cylinder heads of prematurely check static line.

 

[Fun Times]

Re: 496 Mag - Blown motor at 9 hours

How is the exhaust plumbed? thru transom exhaust or down thru the y pipe? Does it have the silent choice exhaust system?

They may need to check to see if the exhaust elbows are at the specified distance above the waterline by referring to the measuring exhaust elbow height section in the service manuals.