please help me with the knocking sound inside the engine

baodj1987 · Jul 10, 2013

hello folks,
this is my problem, I rode my boat and when I suddenly speed up, I hear the knocking sound inside the engine really lound, I was really worry and shut down the engine and let the police pull us to the launch.
and I tear down the engine, I saw the scratch like a letter inside the engine block

I dont know which thing cause these scratch. and is it serious problem?
please help me,
my engine is volvo penta aq 125a
thank you

rickryder · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

These look like some grinder marks of some kind....I doubt that is from your knocking noise....what worries me more is the heat discoloration on the crank @ the bearing cap. If it was a deep rattle knock sound I would bet you have bad main or rod bearings....only way to tell is start tearing it down and look at the bearings....

baodj1987 · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

rickryder said:
These look like some grinder marks of some kind....I doubt that is from your knocking noise....what worries me more is the heat discoloration on the crank @ the bearing cap. If it was a deep rattle knock sound I would bet you have bad main or rod bearings....only way to tell is start tearing it down and look at the bearings....

I have not much knowledge so let me ask question: how can I know if the main or rod bearings is bad and need to replace? I just need to replace the bad things or I have to replace all the piston, rod and cylinder block as well? if you give me some pic for more details, I will really appreciate that. thanks

Silvertip · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

Did you think to look at the water temperature and oil pressure gauges when the noise appeared? Those gauges are there to warn you of impending issues but they only do that if you look at them and understand what they are telling you. If oil pressure is low bearings are the likely cause - not the oil pump. The only way you know is to mark the bearing caps and remove them. The MUST go back the same way you remove them so mark them as to which cylinder and which orientation they came off.

rickryder · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

Silvertip is spot on with the above post. Your main bearing caps are the ones attached to the block...the rod caps are on the connecting rods and piston assys. Mark the caps and remove them.. under the cap will be the bearing,check to see if it's worn or gouged. post pics of what you found....Most cases if the main bearing is worn badly you will need to do something to the crankshaft.....may just need to be polished....turned down and oversized bearings used or replaced....lets not jump too far ahead....see what you have. Did you find any metal in the bottom of the oil pan? Copper colored metallic flakes?

here is a google search showing bad bearings https://www.google.com/search?q=bad...Xh4APEooDgAw&ved=0CAkQ_AUoAQ&biw=1680&bih=925

baodj1987 · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

thank you guys..... your advice helps! get in work now......

baodj1987 · Jul 10, 2013

Re: please help me with the knocking sound inside the engine

rickryder said:
Silvertip is spot on with the above post. Your main bearing caps are the ones attached to the block...the rod caps are on the connecting rods and piston assys. Mark the caps and remove them.. under the cap will be the bearing,check to see if it's worn or gouged. post pics of what you found....Most cases if the main bearing is worn badly you will need to do something to the crankshaft.....may just need to be polished....turned down and oversized bearings used or replaced....lets not jump too far ahead....see what you have. Did you find any metal in the bottom of the oil pan? Copper colored metallic flakes?

here is a google search showing bad bearings https://www.google.com/search?q=bad...Xh4APEooDgAw&ved=0CAkQ_AUoAQ&biw=1680&bih=925

So I torn down the main bearings and found scratches on it and very deep cuts as you can see. Might it be the problem guys?

do I need to polish the crankshaft?
Also, how do I remove the pulley key and the pulley from the crankshaft?
this is the pulley and I dont know whats the trigger for

the trigger in the red circle

thank you guys a lot

baodj1987 · Jul 11, 2013

Re: please help me with the knocking sound inside the engine

Hello.... Please help me!

rickryder · Jul 11, 2013

Re: please help me with the knocking sound inside the engine

My best advice is to get yourself a repair manual for your motor to see what tools you will need to remove the crank pulley....most times you need a gear puller that you can get from your local auto parts store....most of them have a loan a tool program. Without seeing pics of the crank itself hard to say whether it can be polished or if it needs to be turned down by a machine shop.

Maybe someone with more knowledge of the Volvo Penta engines will chime in....I'm more a Merc guy

What do the rest of the bearings look like? You should post pics of all of them.....rods and mains.

baodj1987 · Jul 11, 2013

Re: please help me with the knocking sound inside the engine

Quick update. I found the problem after tearing the engine apart. It is the #4 cylinder corrosion issue. the corrosion is so bad, the valve actually broke and damage the cylinder as well. So I think I need to replace the whole cylinder head. The 4 exhaust pipe is also in bad shape. So do you guys think i should replace the exhaust as well or just the head? Thanks much guys.

betroncm · Jul 12, 2013

Re: please help me with the knocking sound inside the engine

You need to figure out what caused that to break. Is the valve stem frozen on the head? Do you have a leak into the cylinder causing the corrosion? Hows the piston and cylinder wall? Did your engine jumped out of time and your piston hit the valve?

DCinVA · Jul 14, 2013

Re: please help me with the knocking sound inside the engine

With that kind of damage I would be looking at buying a remanufactured short block.
Probably a lot cheaper than trying to fix what you have.

please help me with the knocking sound inside the engine

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