Engine failure... Trans caused?

[Brock Chobar]

Hey Guy's!
So I have a 1992 370DB with twin 7.4 (454) mercruisers and Hurth 63 trans. A few weeks ago on a short run at about 2500 RPM we got a sudden bad shake in the boat from the Starboard side. At first my thought was "lost a trans". Throttle back and took out of gear. Checked trans oil, leaks ect. Had a slight tick but nothing bad. Back in gear and shook right away so I easly went from forward to reverse a few times thinking I had seaweed on the prop... but nothing surfaced. So we took off again and all seemed good. Yesterday we went out for a sea trial after tune ups and oil changes... Boat ran awesome! until we gradually lost power on starboard and couldn't get over 15mph. Turned out to be a lost engine.. Rod knock real bad. SO>> My question is before I drop in a new engine can a bad transmission cause engine failure?? If the trans is/was locking up? Has anyone had this issue? Its obvious the engine is bad now an the sake could have been when it spun a bearing but ive never heard odf a bad engine bearing causing a shake that bad although its possible. Last thing I want to do is drop an engine in and repeat.

Thanks! Brock.

 

[LG111]

Can't really see a connection between trany and engine failure. It's just a shaft from the engine to trany. If a trany went bad on its own, I can see the engine ripping it apart further but not the other way around. Especially via the tourq a 7.4 puts out.

 

[PlayDate]

The "shaking" is more likely due to a series of cylinders not firing. It becomes far more pronounced when the engine is engaged to the transmission.

The more interesting question is what caused the engine to generate a rod knock. To spin an engine bearing requires the engine to lose oil pressure and usually overheat. I am curious what you find when that engine comes out.

 

[Brock Chobar]

Yeah its a weird one... Ive been a mechanic my entire life so I know what causes bearing failure but its just odd how it happened. Always regular oil changes ect... Was running good then got the shakes. When that happened oil pressure seemed fine. Yesterdays run was an even better run. Just did major tune ups and all so we went for a quick trial. speed and RPM was better than before and she was doing great! For whatever reason we lost a bearing. Bad rod knock and it also must have killed a cam bearing because its barking pretty good too. Timing is at 8deg BTDC with all new ignition stuff. Bust Out Another Thousand...s

 

[Brock Chobar]

Now that im thinking about it I don't know if the purple/white ignition wire was ever grounded when the timing was set. I was working on the other engine when the starboard was being timed. If the timing was set NOT in base mode could that cause a misfire so bad that it could spin a bearing?

 

[PlayDate]

So you have a Thunderbolt V ignition? Thunderbolt IV's don't have that wire. A Thunderbolt V has a purple/white wire you basically disconnect to set base timing and ground. The purpose of the amplifier is to control the advance. If you didn't disconnect it...you may be off a few degrees at idle. The amplifier would restrict high advance based on feedback from the knock sensor.....so no....that wouldn't cause an spun bearing. How many hours were on the engine?

What may have caused it to fail is water ingestion. It could have just been a coincidence with the timing of the work you did and a manifold gasket/manifold failing. That would explain most of the symptoms and highlight why the oil pressure didn't drop and the engine did not overheat. Water ingestion would explain the rough/great running and the damage to the crankshaft.

 

[Brock Chobar]

I think it is Thunderbolt IV? No knock sensors. I have the in distributor sensor with 3 wires from it and module mounted to the riser. I think... the engine has 400ish hours. They were replaced about 7 years ago from a previous owner.

I thought of water injestion too. We never pulled the intake but it could have a bad riser or manifold I guess. I guess ill know for sure when we pull it next week.

Do you happen to know the timing steps? I know its to be at 8 BTDC. Does a thunderbolt IV require a jumper or anything special? Thanks a million in advance for your help Sir!

 

[PlayDate]

A Thunderbolt IV does not have a purple/white wire and is timed as an old school distributor. Nothing to ground or jump.

A Thunderbolt V has a knock sensor next to the starter and a purple/white wire that goes to the distributor. Most people see no difference with a timing light if the purple/white wire is grounded or connected at idle. That said, the reason you set base timing with the wire grounded is so when you reattach the wire it is working from the new base timing and advancing from there.

Based on your model year....you could have either one so check for the wire and go from there.

8 BTDC is fine.

7.4s are tough engines.....most of them live a hard life and die of water ingestion. All it takes is an exhaust manifold water leak back into the head and the engine is done. It can't compress water. I doubt the leak came from the intake manifold. If it was a big leak....the engine would have thrown a rod through the block.

Happy to help.

 

[ttmott]

Engine failure due to transmission issue? Doubtful.
Vibration due to engine to trans coupling failure? Probable - loud knocking noise at idle can sound like rod bearing failure.
Failure due to water ingestion While Running?? Doubtful.
Failure due to water ingestion After Shutdown then Startup?? Probable; spark plugs and oil will reveal that.
Is oil in pan milky and oil/water mess (milkshake) under the valve covers? Bearings lost due to water in oil..
Crankshaft issue - Slightly Probable - Check end play on crankshaft and rotate engine by hand left and right. If replacement engine had cast crankshaft (not Mercruiser engines with forged crankshaft) - possible broken crank. If crankshaft has more than .080 inches movement in and out then thrust bearing issue. Pull oil filter and cut open; metal particles in filter pleats will tell a story.
Connecting rod bolt failure - Slightly probable; poor engine assembly if rebuilt - Remove oil filter and cut open like above.
Are motors full closed loop cooled, partial closed loop cooled, or raw water cooled? Boat used in saltwater or lake water? Will help isolate if failure due to block, manifolds, head corrosion or gasket failure or not.

 

[CliffA]

for what it is worth and since you are a mechanic you probably know this, but I recently had a fairly loud knocking sound from my starboard engine which sounded like a rod knock ( I'm an old 'shade tree mechanic' so I am familiar with various engine sounds).....the engine seemed to run smooth.....the problem turned out to be a fuel injector not firing....replaced the injector and the knock went away.....

might be something to look into...

good luck...

cliff

 

[PlayDate]

Cliff,

Unless he re-powered....I believe he has carburetors. You are right though....with injectors....they can cause all manor of sounds when they act up. They can make an engine sound like a hammer hitting the transmission when it is in gear at idle.

At least from the symptoms including the complete loss of power....I believe something more catastrophic has occurred. I would probably pull the plugs and run a compression check on the motor while waiting for it to be pulled. There is no downside to it and it probably will provide a clue to what is wrong. For example....water on the plugs....oil on the plugs. Even checking the oil for being milky may provide a clue. I worked on one boat where we pulled a plug and water poured out. If the compression check (and oil) is okay.....then I would do a few more diagnostics before I pulled the engine.

If it is water intrusion....the other engine won't be far behind in terms of experiencing the same problem.