Mercruiser 4.3 Knocking noise when idle

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adam125

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hi all new to this site! it looks very helpful so thought id give it ago. I have mercruiser 4.3 v6 1998 it is running perfect no temp issues oil pressure all good. They only problem I have is an annoying knocking sound when it sits on idle seems to becoming from the timing chain area? I have removed the belt to ensure it was not the water pump or alternator ect... noise was still present I have also pulled each spark plug individually and the noise is still there? any help would be much appreciated thankyou

Adam :)
 

achris

More fish than mountain goat
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Re: Mercruiser 4.3 Knocking noise when idle

Hello Adam and welcome to iBoats.

Noises are the most difficult to diagnose 'over the phone'. Only you can hear it, and you tell us what you hear. And here's where it gets messy. Your interpretation of a noise might be a 'tick' for example. Now I have in my mind what I think of as a tick, but the noise you call a tick, I call a click. See where I'm going?

So, some questions about your 'annoying knock'...
1. Does it come on as soon as the engine drops to idle or does it develop as the engine idles? If it develops, how long does it take?
2. Is it there regardless of which gear the drive is in (neutral/forward/reverse)?
3. If you lift the engine speed without load (drive in neutral), is it still there?
4. At what engine speed does it disappear?
5. When it disappears, does it go away gradually or immediately?
6. What frequency is the the at? (half engine, full engine)... ie, is it timed to the engine?
7. If the noise is there at idle without the drive in gear, have you tried it on the trailer?
8. If so, and it's still there, have you run the engine with the drive removed (to rule the drive out)?

Chris....
(8 questions... ;))
 
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adam125

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Re: Mercruiser 4.3 Knocking noise when idle

Hi Chris thank you for your reply very nice of you :)

right
1, It develops when in ideal after around 5 mins of running
2, The sound only appears when in neutral
3, As soon as you up the revs it seems to disappear
4, I'm not 100% of the engine speed but seems to be as soon as you put into gear sound disappears
5, When it disappears it is gone immediately
6, It seems to be with the frequency of the engine
7, I have not tried it on the trailer
8, also have not tried it without the drive

I have tried to locate the sound using a piece of wood I am sure it is coming from the front of the engine by timing chain cover :( I have used the boat with the noise for a good few hours an no problems have developed I would just like to know where it is coming from very frustrating :( also I don't want to get caught out an cause major damage to the engine.

Thanks again for your reply chris :)
 

achris

More fish than mountain goat
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Re: Mercruiser 4.3 Knocking noise when idle

Adam,

Had it been an in-line 6 I would know exactly what it is, but I have not heard of the V6s developing gudgeon (piston pin) knock, but that's what it sounds like.

No sound when it's idling before warmed up and disappear as soon as load is applied.... Yep, sounds like a piston pin. Any chance of videoing the engine running with the sound and posting to y-tube?

Chris.......
 

Bondo

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Re: Mercruiser 4.3 Knocking noise when idle

Ayuh,... What year is the motor,..?? Old enough to have a mechanical fuel pump,..??

Chris, correct me if I'm wrong,...
If a wrist pin is loose, won't it go Quite, when the spark plug wire is pulled off that cylinder at idle,..??
 

achris

More fish than mountain goat
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Re: Mercruiser 4.3 Knocking noise when idle

Ayuh,... What year is the motor,..?? Old enough to have a mechanical fuel pump,..??

Chris, correct me if I'm wrong,...
If a wrist pin is loose, won't it go Quite, when the spark plug wire is pulled off that cylinder at idle,..??

1998, no mech fuel pump....

:facepalm:... Completely forgot about the plug wire trick... good one.. Thanks Bond-o, I owe you (another) one.... :D

Adam... You understand that?

Chris......
 

Bondo

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Re: Mercruiser 4.3 Knocking noise when idle

1998, no mech fuel pump....

:facepalm:... Completely forgot about the plug wire trick... good one.. Thanks Bond-o, I owe you (another) one.... :D

Adam... You understand that?

Chris......

Ayuh,... You don't owe Me, Nothin' Chris,...

This is a 2 way street ya know,... ;)

But,... I'd settle for a Handshake, next time yer in Vermont skiin' or whatever,...
I can drive that far to buy ya a drink,....
 
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Re: Mercruiser 4.3 Knocking noise when idle

I don't think that a wrist pin "slap" condition can be detected by removing the plug wire (OP said he tried that) - also it would still be noticeable at higher speeds.
Assuming it's not a low idle oil pressure - I'm wondering if it is a timing chain/gear/tensioner "problem"?
 

achris

More fish than mountain goat
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Re: Mercruiser 4.3 Knocking noise when idle

No tensioner on these engines. And if it was a timing chain, it should get worse as the engine speed increases. I had an old in-line 6 (first engine in the boat) and that had a significant gudgeon knock, and it disappeared as soon as the engine was off idle.

Chris...
 
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Re: Mercruiser 4.3 Knocking noise when idle

Yes, you're correct on the timing chain. As you said earlier it's very hard to diagnose noises over the phone - this noise doesn't seem to fit under any of the usual causes:(
I did find this link - don't know if info is correct - but #6 outlines procedures for identifying wrist / gudgeon pin looseness.

TROUBLESHOOTING ENGINE NOISES
 

adam125

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Re: Mercruiser 4.3 Knocking noise when idle

Thanks again for your replies! All very helpful ! I will video it for you tomorrow you guys can have a listen an see what you think? I just think if was the pin do you think the engine would eventually come to a crash ? I have been using the boat quiet frequent lately making the most of this nice weather we have been getting in the uk! I did try tonight to get the timing chain cover off but quickly realised it looks like the engine would need to be pulled to get the harmonic balancer off ? Am I right in saying this?

Thanks a lot guys your help is much appreciated
 

achris

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Re: Mercruiser 4.3 Knocking noise when idle

To remove the timing cover the engine does need to come out. Not only does the balancer need to come off, but also the oil pan and the water pump...

Chris....
 

adam125

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Re: Mercruiser 4.3 Knocking noise when idle

Ok il get a video for you guys today! If it was a gudgeon pin would it be ok to to use the boat for the rest of the season ? An get it sorted in the winter?
Adam ..
 

adam125

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Re: Mercruiser 4.3 Knocking noise when idle

Ok il get a video for you guys today! If it was a gudgeon pin would it be ok to to use the boat for the rest of the season ? An get it sorted in the winter?
Adam ..
 
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Re: Mercruiser 4.3 Knocking noise when idle

Ok il get a video for you guys today! If it was a gudgeon pin would it be ok to to use the boat for the rest of the season ? An get it sorted in the winter?
Adam ..

IF it's a pin, ( or any other worn internal component )then it will get worse with use, but impossible to say how long it will hold up :(
If you do use it, be gentle!
 

bonzoscott

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Re: Mercruiser 4.3 Knocking noise when idle

If the noise goes away after idle, just don't run it at idle :laugh:, enjoy remainder of the season and keep an eye out for a replacement.
 
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Re: Mercruiser 4.3 Knocking noise when idle

Mercruiser knock - YouTube
Check out the video guys ignore the missing belt an water pump thanks alot for your help!

Good video :) I'm not sure that the noise actually goes away when revs are increased, I think it just changes its sound (it sounds a little uneven). One thing I noticed is that the noise appears to be at 25% of engine speed, so that does indicate a problem in one cylinder.
Have you done a compression test? May just be a bad valve, although noise is a little "heavy" for that. Worth checking out.
 

Volphin

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Re: Mercruiser 4.3 Knocking noise when idle

Sounds to me like a lifter issue or a weak valve spring. It's missing the solid KNOCK of a rod.
 

adam125

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Re: Mercruiser 4.3 Knocking noise when idle

Ok thanks for your input I think a compression test could be the next thing! If it was a lifter or a weak valve spring is it ok to run for the rest of the summer? It still seems to be coming from the timing chain though? Could it be just a worn chain?

Thanks again
 
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