Thoughts needed - knocking when in gear?

jmunro123

Member
Feb 2, 2008
370
Gran Bend, Great Lakes
Boat Info
2004 Sea Ray 390 Motor Yacht
Engines
8.1's
Today when I put the boat in gear while at idle (Starboard side) started knocking or kinda clanking along with a vibration strong enough I could feel it on the bridge. But, when the RPM was increased the noise and vibration when away. Also in reverse the sound is there but not as strong.

Is it safe to take out on the water , any idea what it could be?
 
Sounds like your starboard engine is misfiring on one or more cylinders. It will cause the springs in the damper plate to rattle and the misfiring would cause the vibration you feel. 8.1s are known to eat up spark plugs fairly quickly. When were they last changed. Are the plug wires, cap and rotors original to the boat. they would be overdue to be changed if they are. If those don't cure the problem then a compression test is in order.
 
The plugs have never been replaced, we put about 100 hours on a year. OK, thanks for the idea I will check it out.

So if that is the issue then using it today won't be a problem right.
 
The plugs have never been replaced, we put about 100 hours on a year. OK, thanks for the idea I will check it out.

So if that is the issue then using it today won't be a problem right.

I had a similar problem which turned out to be a chaffed plug wire arcing to the ex manifold. That motor also wouldn't hit full revs.

I wouldn't run it until you know exactly what the issue is. Having a stray spark from a chaffed plug wire could lead to an explosion where fuel vapours are present.
 
Smart move.

The sig pic is not actually a marina, but a place to pull up to grab a bite and a drink close to the city where i live. Was a new addition to our city about 3 years ago.
 
Very smart move. We had the same problem. The knocking is the slap plate or sometimes called a flex plate by the tranny. We ended up with a bad cylinder and had the engine pulled and rebuilt last winter. It could be something as simple as a plug or plug wire. Be warned: a set of plug wires are close to $500 for the 8.1s
 
If you shop around 8.1 plug wires can be had for just over $100. OEM AC Delco
 
I had heard that as well. I was corrected and told that the connectors are different from the 8.1 truck engine to the Mercruiser. Is there any validity to that?
 
I had a similar issue on my 340 which turned out to be a loose prop nut. When the boat was in forward gear the prop would migrate to the end of the shaft against the nut and vibrate. When in reverse it came forward toward the strut and did not vibrate as much. Met worth checking before spending too much on replacing parts. I changed plugs and wires without solving the issue which was discovered during winter haul out.

Another possible issue causing a misfire could be plugged injector. Had this happen after paint from cool fuel module got into fuel. Very easy to pull and send off for rebuild.
 
I bought Volvo Penta wires for their 8.1 motors, an exact match for $58 dollars a set from Cecil Marine online.

Volvo Penta part# 3861295
 
I bought Volvo Penta wires for their 8.1 motors, an exact match for $58 dollars a set from Cecil Marine online.

Volvo Penta part# 3861295

Is this for a set of 8 wires? I looked it up on Cecil's website but it's hard to tell how many wires are in there.
 
I have the same situation going on with my port motor/gear box and also after all the experts looked it over and I had Merc replace all injectors, fuel module and cleaned and reassembled. I still have a slight Knock now but the techs in my area assure me that it is just "noise" at this point. Honestly I hate it but i dont see throwing money at it if it isnt broken!
It does go away as soon as I go past 700ish rpm and only one motor is doing it.
c
 

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