Intermittent knock - VP 5.7 Gi300-J

[Leo87]

Hi group, I have a VP 5.7 Gi-300-J from 2010. I've had it since new and it now has 230 hours. It runs really well except yesterday a knocking noise started happening. I shut it off as soon as I heard it and did a visual inspection. I didn't find any obvious problems.
I then re started the engine and the noise was still there. At idle I hear the knock about 140 times a minute.
I came back home at no more than 1500 RPMs just in case it was something serious. Turning the steering back and forth didn't make any difference. I did spray some water on the steering pump pulley and the noise got lower for a few seconds. The other pulleys are not at arms length so I couldn't spray them at that time.
Unfortunately I didn't have a wrench to temporarily remove the belt to see if the noise would go away, indicating a bad pulley, etc.
After about 15 minutes I stopped for about an hour. Once I started it again, the noise was still there but it went away after about 10 minutes. For the last 5 minutes or so I ran it at 3000 RPMs. I got back to my dock without any incident.
Temperature and oil pressure were normal at all times.
This morning I started the engine 5 times and the noise is NOT back. I wanted to remove the belt to see if the noise would go away and further troubleshoot.
Below are the before ("with knock"), and after ("without knock") videos. Any idea what the noise could be? All pulleys are original. I am thinking belt tensioner but maybe one of you had this same issue in the past and can confirm what the problem is. I rather replace the part now, before it breaks and possibly cause other issues.


Thank you in advance for any help you can give me.


2010 VP 5.7 Gi300-J

 

[alldodge]

There is a white spot on the serpentine belt and appears to knock about every time I see the spot. This gives me the thought it is doing it about every 4th revolution (thinking about the length of belt). There seems to be slight more movement of the belt moving back n forth on the water pump pulley when it was knocking then when it wasn't. Unable to say for sure because the no knock vid was more stable.

Is this a stern drive of inboard?

 

[Leo87]

Yeah, I was freaking out when I took the first video thinking it was something internal. If it happens again I'll make sure to make it longer and zooming into each pulley.
It is a sterndrive (duoprop).
Thank you for your help.

 

[alldodge]

Remove the belt and see if you can find any play on the pulleys

 

[Leo87]

Ok, I'll try that and report back in a few hours. Thanks again.

 

[Lou C]

Agreed, that does not sound like an internal noise but something with the belt/pulleys. Check each accessory bearing and the idler pulleys. Also look at the harmonic balancer on the crankshaft, make sure it is not wobbling because that can cause the belt to wobble around like that.

 

[Scott Danforth]

check the belt tensioner and idler

 

[kenny nunez]

If the belt is worn to the point where the tensioner is at it’s limit there will be a knocking sound as the tensioner is bouncing on the stops. I experienced this on a truck I had ,

 

[Leo87]

Thank you all. Valid points and I'll keep them in mind.
I removed the belt and only the one on the belt tensioner has some play. The rest look and feel fine.
The belt only has 80 hours and it is two years old. I used a belt gauge and it passed the test. I'll measure it just in case it is stretched.
I am not sure what to replace first. I am tempted to just get the tensioner w/idler since it has some play. I don't really want to go out like this again.

I also noticed the alignment marks on the tensioner are not lined up. I am not exactly sure what this means but must be there for a reason.

Here is the picture...

 

[alldodge]

That doesn't mean much, it's the alignment marks but shows wear. There is a spring inside.

You sure you have the correct length belt?

 

[Leo87]

Yes, I have belt 21132390. Keep in mind this only happened yesterday for a few minutes. I went ahead and ordered a new belt tensioner with idler. I should have it by Thursday. I'll report back once I put a few hours on it. Hopefully that was it.
Thank you everyone for all your help.

 

[Scott Danforth]

if the belt is too long, new idler wont help that.

 

[alldodge]

Yes, I have belt 21132390.

That shows to be the correct

https://www.marinepartseurope.com/en/volvo-penta-explodedview-7747710-30-22064.aspx

 

[Leo87]

UPDATE: I've used the boat for a few hours and so far no issues since I installed the new belt tensioner. Thank you all for your help.

 

[dearmosd]

Just out of curiosity did you change the belt too? If so did you replace OEM or aftermarket.

 

[Leo87]

I used the same belt since it was only 2 years old, with 80 hours of use. Still no issues.