Engine knock

[Nick duBois]

Hello all,
A few weeks ago My oil pressure safety switch sprung a leak and I lost most of the oil in the crankcase. I was just docking when the engine got louder and there was quite a bit of banging going on. I looked at the oil pressure gauge and nothing..... I shut the motor down at once and did some troubleshooting. Put new oil in, found and fixed the leak. I did another oil change. It is much quieter but i have a few residual issues.
1. oil pressure at startup is 40lbs, when warm 20lbs at idle 25 lbs under load at
1000 rpm
2. there is a new noise in my atomic four i have never heard. I have had the
boat ten years and i know what it normally sounds like. something is not
quite right...a knocking noise that was not there before.
3. Engine is starting to burn oil slightly. I can hardly see it but I can smell it
Here is a link to the video and sound. https://www.youtube.com/watch?v=BmGLbmUHhp8
What do you folks think

[joe_db]

Hello all,
A few weeks ago My oil pressure safety switch sprung a leak and I lost most of the oil in the crankcase. I was just docking when the engine got louder and there was quite a bit of banging going on. I looked at the oil pressure gauge and nothing..... I shut the motor down at once and did some troubleshooting. Put new oil in, found and fixed the leak. I did another oil change. It is much quieter but i have a few residual issues.
1. oil pressure at startup is 40lbs, when warm 20lbs at idle 25 lbs under load at
1000 rpm
2. there is a new noise in my atomic four i have never heard. I have had the
boat ten years and i know what it normally sounds like. something is not
quite right...a knocking noise that was not there before.
3. Engine is starting to burn oil slightly. I can hardly see it but I can smell it
Here is a link to the video and sound. https://www.youtube.com/watch?v=BmGLbmUHhp8
What do you folks think

I think the engine will need to be rebuilt to solve these issues. As a temporary fix, try using heavier oil.

[ndutton]

I think the engine will need to be rebuilt to solve these issues. As a temporary fix, try using heavier oil.

Agreed but with this caveat: the longer you put it off the more risk of greater damage.

Case in point
When I acquired my spare engine it ran but with a deep sickening knock like John Henry was swinging his hammer inside the engine. A tear down and analysis found the #2 rod bearing was completely gone, ground into hamburger and glommed onto the oil pump pickup screen. I'm sure it happened over time, started small, maybe even an episode like yours but the deterioration continued. Additional damage was an oval rod end and a bent crankshaft. I believe neither would have been issues had the initial problem been dealt with early.

[Nick duBois]

I can confirm that when I pulled the plug wires off one after another the knock pretty much stopped on the third cylinder. There was no difference in cylinders 1,2 and 4

[joe_db]

I can confirm that when I pulled the plug wires off one after another the knock pretty much stopped on the third cylinder. There was no difference in cylinders 1,2 and 4

I would take the engine home and see what you have.

[Nick duBois]

I would take the engine home and see what you have.

Yeah i think that is my best option. Im going to use it lightly at low rpm for the next month. then when the boat comes out of the water the motor comes out of the boat. Funny it does not always make that sound. It needed a paint job anyway. Pulling an atomic four out of a Catalina 27 is going to be no easy task....not much room on either side.

[romantic comedy]

Yeah i think that is my best option. Im going to use it lightly at low rpm for the next month. then when the boat comes out of the water the motor comes out of the boat. Funny it does not always make that sound. It needed a paint job anyway. Pulling an atomic four out of a Catalina 27 is going to be no easy task....not much room on either side.

You might consider leaving that plug wire off for a while. Technically you should ground it.

[wristwister]

Now if you go to my engine rebuild thread, you'll see I also had a knock in an A4, which ended up with a golf ball size hole blown through the block by an errant connecting rod.

But I'm curious, in the OPs case, the knocking goes away when he disconnects #3 spark plug. If the problem were a crank or piston bearing going out, how would removing spark make it go away? The crank is still turning, the piston is still slapping madly up and down. Is it just because the power is removed from the power stroke?

[romantic comedy]

Now if you go to my engine rebuild thread, you'll see I also had a knock in an A4, which ended up with a golf ball size hole blown through the block by an errant connecting rod.

But I'm curious, in the OPs case, the knocking goes away when he disconnects #3 spark plug. If the problem were a crank or piston bearing going out, how would removing spark make it go away? The crank is still turning, the piston is still slapping madly up and down. Is it just because the power is removed from the power stroke?

Without the combustion there is much less force on the piston-rod-crank. It is just going up and down and round and round. Combustion is a huge force on the piston, any play in the crank journal might make the knock.

[wristwister]

Without the combustion there is much less force on the piston-rod-crank. It is just going up and down and round and round. Combustion is a huge force on the piston, any play in the crank journal might make the knock.

OK, that makes sense. But that has me thinking that when my daughter and I were motoring her boat to it's new home with the engine knocking away, we might have actually made it if we pulled the plug from the bad cylinder and motored on 3 cylinders. Good lesson learned, and probably applies to any piston engine; gas, diesel, boat, car, whatever.

[Nick duBois]

funny thing... under load in gear there is no knock at all. At 1500rpm i can not detect a thing. I went down and listened for it while someone else had the helm and nothing. Only in neutral or when reducing the rpm can it be heard. Can anyone venture a guess as to why???

[Mo]

Nick, if you like you can give me a call and I might be able to drive over and have a listen to it for you. 483 8524

[Nick duBois]

Thank you Mo that would be awesome. I will give you a shout tomorrow and we can pick a time that works for you.

[Scott312]

Suspense is killing me. What did you find?

Mine is knocking too, sounds almost identical.

[jayw]

Knock, knock

I wondered why it was making so much noise right at the end....