4.3 mercruiser enigne tap/knock?

[Nrizzo20]

Hey everyone,

I have a 1993 bayliner capri with a 4.3 mercruiser alpha one gen2 outdrive. 2 weeks ago the boat was started for the first time this year. I took it out a two or three times in the lake without any issues, approximately 30 minutes each time. Last week I took the boat out again for about 2 hours. Most of the time we were putting around fishing and what not. Before we trailered her back up we did a few laps around the lake for about 10 minutes. We ran at about 4800 rpms, never losing power. I came down to idle and started to hear a tapping sound. The sound was at idle, at 1500 rpms and again at 3000 rpms. Consistent the whole time. My oil pressure was still spot on, around 40psi at idle, temp was perfect and the engine never made a hiccup. Just this god awful noise that no boater ever wants to hear.

I left the boat cool down and went to diagnose the problem the next day. I ran it in the driveway with muffs and the noise wasnt there... until about 5 minutes later it came back. The noise sounds like its coming from cylinder #6 but I havent had a chance to dig into it with a busy work schedule this week. Just curious if anyone else had any other suggestions. I took a video to make it easier for anyone to help! thanks for your help guys!!!

https://www.youtube.com/watch?v=3cilHuMQbYw&feature=youtu.be

 

[Bondo]

Ayuh,.... Welcome Aboard,..... Don't sound good, at all,.....

Maybe start it up, induce the knock, 'n pull one spark plug wire at a time, 'n see if you can isolate which cylinder,..??....
Pull the valve cover off whichever ya find in the plug wire test, 'n go from there,...

10 minutes at Wot, is a L oo nnn gggg time,... atleast in My book,....

 

[Nrizzo20]

Ayuh,.... Welcome Aboard,..... Don't sound good, at all,.....

Maybe start it up, induce the knock, 'n pull one spark plug wire at a time, 'n see if you can isolate which cylinder,..??....
Pull the valve cover off whichever ya find in the plug wire test, 'n go from there,...

10 minutes at Wot, is a L oo nnn gggg time,... atleast in My book,....

I'm a bit new to the boating scenario, but when I had it WOT the tach was reading nearly 6000 rpms. What will pulling each spark plug wire do while it's running? I will pull the valve covers tomorrow before work. Thanks for the reply!

 

[achris]

With the engine running, if you pull the lead off any cylinder, it 'unloads' that cylinder. If it's a main or con rod knock, pulling the lead will make the noise go away, or change significantly. That's how you find which cylinder it is. Pulling the valve covers you're looking for broken, loose springs, etc.

Chris......

 

[Bondo]

but when I had it WOT the tach was reading nearly 6000 rpms.

Ayuh,.... Rated rpms at Wot is 4800 rpms,....

 

[crazy charlie]

to isolate a tap/knock etc you can do this simply with a wood broom stick.Put the end of the broomstick where you think the tapping is coming from.Put the other end of the broomstick in your ear.You WILL hear where it is coming from by touching the en to all of the spots that you think it is coming from.You may also want to try the pipe coming off the back of the riser.Exhaust flaps can fool you by tapping/clicking at various times.Charlie

 

[Nrizzo20]

With the engine running, if you pull the lead off any cylinder, it 'unloads' that cylinder. If it's a main or con rod knock, pulling the lead will make the noise go away, or change significantly. That's how you find which cylinder it is. Pulling the valve covers you're looking for broken, loose springs, etc.

Chris......

I also read an exhaust manifold gasket could make this noise. Would the noise turn from a tap to a knock in that situation??

 

[Nrizzo20]

to isolate a tap/knock etc you can do this simply with a wood broom stick.Put the end of the broomstick where you think the tapping is coming from.Put the other end of the broomstick in your ear.You WILL hear where it is coming from by touching the en to all of the spots that you think it is coming from.You may also want to try the pipe coming off the back of the riser.Exhaust flaps can fool you by tapping/clicking at various times.Charlie

I had a friend come over and he tried isolating the sound with a piece of hose. He said the noise sounded top end like a lifter. But I'm getting different responses from the video I posted.

 

[crazy charlie]

try the broomstick.You will be amazed how well it works.

 

[JaCrispy]

Easiest thing to do is pull the valve covers and inspect the rockers. Could have a loose rocker or bent push rod.

 

[Maclin]

I heard valvetrain noise AND a rod knock, ouch.

 

[sub2010ss]

I agree, some type of solid stick for sound isolation works great. Sounds like a lifter to me.

 

[H20Rat]

I hear rod knock... What prop is on that setup? It is drastically under propped if it hit 6000 rpm, and that 4.3 is lucky it didn't disassemble itself on the water at that rpm!

 

[Nrizzo20]

I hear rod knock... What prop is on that setup? It is drastically under propped if it hit 6000 rpm, and that 4.3 is lucky it didn't disassemble itself on the water at that rpm!

The prop I had was a 14 1/2 x 17p. I bought the boat with that set up. Since then I changed to a 14 1/2 x 21p.

 

[Nrizzo20]

So a little update from a few weeks ago. I'm still lost here!! I pulled the valve covers, replaced all lifters and inspected everything else. Push rods and rocker arms all seemed fine still. I replaced gaskets and put everything back together. (Torqued everything to specs). Started the boat and the noise came back immediately. I pulled each spark plug wire one by one and the noise did not change or go away what so ever. I am lost on this one!

 

[jopes]

you can also put a timing light on it. See how frequently it knocks compared to the light flashing. But it sounds like a lower end problem vs's a upper end issue.

 

[Nrizzo20]

Well from the post I made a couple hours ago, the results have changed. I got the engine to running temp. When I pull the #6 spark plug wire the noise gets a lot louder. More than a knock over anything. When I pull all the other wires there isn't any change