Marine engine knocking

[Donald]

Looking at a boat (to maybe buy) with an 8.1L GM based engine that is knocking. Engine went into limp mode. Engine still starts and runs. So obviously worst case is a new or reman engine. The boat price has an engine replacement factored in. What other engine issues besides something like a broken rod would cause the engine to have a knocking sound but still run. Some engine failures might not even allow for the core to be usable.
I have asked but have not been told if engine ran out of oil or overheated.

 

[wpod]

How is the drive live ? A new crate motor would be the best choice unless you have access to a great rebuilder / machine shop. Plan on spending lots of $$$ for top quality. Time on the water is too valuable to go cheap.

 

[Trav]

Looking at a boat (to maybe buy) with an 8.1L GM based engine that is knocking. Engine went into limp mode. Engine still starts and runs. So obviously worst case is a new or reman engine. The boat price has an engine replacement factored in. What other engine issues besides something like a broken rod would cause the engine to have a knocking sound but still run. Some engine failures might not even allow for the core to be usable.
I have asked but have not been told if engine ran out of oil or overheated.

8-10K for the engine alone plus installation and any other parts, that boat has to be pretty cheap. Lots of things can cause a knock, mains, rod, water pump, flywheel. etc.
I bought a 65 Riviera with a bad knock for cheap years ago, it had a dented oil pan in the front that's all.
I could feel it when I put my hand on the pan with the engine running. Changed the pan and no more knock.

 

[Donald]

How is the drive live ? A new crate motor would be the best choice unless you have access to a great rebuilder / machine shop. Plan on spending lots of $$$ for top quality. Time on the water is too valuable to go cheap.

I would either get a new crate engine or buy a reman. I would not get the engine rebuilt. The large engine remanufacturers have expensive machinery that the local engine machine shops cannot afford no matter how skilled they might be.

 

[Trav]

+1 crate motor.

 

[skyactiv]

Remember that GM sells their NEW engines cheaper compared to all other makes.
Why would you buy a reman vs new?

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[Creekside]

Oil sample oil sample oil sample

 

[DriveHard]

Are you sure the knocking is coming from the engine, and not the drive?

There is rotary motion going to the drive at all times the engine is running. You should NEVER run an out drive with the drive in the raised position (that is what the line on the trim gauge is for). You should also NEVER run a drive without being hooked to a water source (muffs).

Does the knocking change if you lower the drive or change when the drive is turned left or right?

Other considerations:
Gimbal Bearing (usually shows as a growling sound)
Outdrive U-joints (usually shows as a knocking sound or vibration)

 

[Donald]

Are you sure the knocking is coming from the engine, and not the drive?

There is rotary motion going to the drive at all times the engine is running. You should NEVER run an out drive with the drive in the raised position (that is what the line on the trim gauge is for). You should also NEVER run a drive without being hooked to a water source (muffs).

Does the knocking change if you lower the drive or change when the drive is turned left or right?

Other considerations:
Gimbal Bearing (usually shows as a growling sound)
Outdrive U-joints (usually shows as a knocking sound or vibration)

The engine went into limp mode. Cannot see that from a outdrive u-joint problem.

Being a 10 yr old engine with water cooled exhaust manifolds, could be water into the engine from the exhaust manifold.

In most cases end of life for a marine engine comes from water intrusion.

 

[mk378]

The engine went into limp mode. Cannot see that from a outdrive u-joint problem.

That's based on a naieve assumption that there must only be one problem with this boat.

The computer usually doesn't care about mechanical failure such as bad bearings.

 

[DriveHard]

The engine went into limp mode. Cannot see that from a outdrive u-joint problem.

Being a 10 yr old engine with water cooled exhaust manifolds, could be water into the engine from the exhaust manifold.

In most cases end of life for a marine engine comes from water intrusion.

Sorry, I'm just trying to make sure you realize a knocking sound could come from other sources on a boat with an outdrive.
Hook a diagnostic tool up the engine and pull the codes.
Pull the dipstick and look for water in the oil.

It is always awesome when folks ask for free advice, and instantly p00-p00 advice they may get.

Wish you the best of luck on the boat, and hope it is an easy fix for you if you choose to purchase it.

 

[Donald]

That's based on a naieve assumption that there must only be one problem with this boat.

The computer usually doesn't care about mechanical failure such as bad bearings.

I am aware that any boat or vehicle can have multiple problems. But I have a feeling that these two issues are related.

 

[thastinger]

I would run away from that unless you plan to keep it a lifetime. Plan on another couple Gs for manifolds/risers and exhaust Y pipe.

 

[Donald]

I would only buy the boat if the price factored in an engine replacement. If it turned out something less than an engine replacement then ahead of the game. Also a new engine in a boat is a good thing

 

[skyactiv]

Does the price reflect the hassle factor of buying such a boat?