Loud, intermittent "knock" when at cruising speed

[K0NPHL1C7]

Hey guys - New to the forum, and new to boating in general. I tried searching but didn't really see anything that mirrored my specific issue.


I have a 2005 Four Winns H180 with a 4.3 GL Volvo Penta (Volvo SX Outdrive). Whenever I open up the throttle and get to about 4200rpm, I hear a single, loud "knock" coming from the bilge area. At first I though t I just had a loose bolt bouncing around somewhere, because it almost sounded like something banging, but it seem to be pretty consistent when it happens. Doesn't sound like an engine knock, and doesn't happen more than once at a time.

Anyone have any ideas?

 

[alldodge]

Howdy
Folks will have a hard time figuring out a knock without hearing it. You say its not from the motor, but being at 4200 rpm would have me thinking of preignition knock or a ping sound. Check you timing

 

[Lou C]

One single loud knock suggests that something is moving around. I recall some time back hearing about loose fuel tanks on these models making noise. With the plastic tanks they use a metal tab that is screwed into the stringer and presses on the tank to hold it. Might want to check that out, I heard about this on the ifourwinns board. You’ll have to pull up the tank hatch and check it out.

 

[K0NPHL1C7]

Oh good call, I haven't lifted that hatch yet to check, that'll be my next step. I was leaning toward it being something moving around. I even found a nut that had come loose from my swim deck and thought that may have been what it was, because it sounds like a nut/bolt might be bouncing up hitting the fiberglass. Just a single loud "bang", not repetitive like a rod knock or anything.

 

[Lou C]

www.iFourWinns.com • View topic - In -Floor Storage Locker/ Ski Locker?????

here's what I could find. Could be this or something else moving. Do you have the battery boxes with that strap that always gets loose? I wonder if so one might be banging against something. I hated those boxes and got a pair of battery trays with the clamp to hold it down. Another idea, what about the transom ladder? Noises are mysterious.
I had the one (or 2) loud bang a few times and what it turned out to be was my gas tank moving because 3 of the 4 wood mounting strips rotted off! When I re-did the deck back in 2006/7 I made new wood mounts out of mahagony that are still solid now, this boat uses an aluminum tank with square mounting tabs that get attached to the mahgony tank mounts.

 

[boomer632]

Hey guys - New to the forum, and new to boating in general. I tried searching but didn't really see anything that mirrored my specific issue.


I have a 2005 Four Winns H180 with a 4.3 GL Volvo Penta (Volvo SX Outdrive). Whenever I open up the throttle and get to about 4200rpm, I hear a single, loud "knock" coming from the bilge area. At first I though t I just had a loose bolt bouncing around somewhere, because it almost sounded like something banging, but it seem to be pretty consistent when it happens. Doesn't sound like an engine knock, and doesn't happen more than once at a time.

Anyone have any ideas?

Does it occur at the same RPM? I heard what I thought was a knock on my 2005 5.0 GXi as well. It was found to be backfiring through flame arrestor due to a lean condition from dirty fuel injectors.