2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

[Hansolo99]

OK I took my boat to the River for a few days and had a great time. The boat ran perfect (2002 Bayliner 195 Capri Sport with a 4.3 V6 and 70 hours on the clock). At 3700 RPM at 42 MPH the boat makes what sounds like a knocking sound and the Tach will drop and jump up and down about 100 RPM. As soon as I back off it's gone. I did not try to go past that. Anybody have a clue????

 

[Hansolo99]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

Anyone??

 

[Maclin]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

Sounds are hard to describe and get lost in translation, some people's knocks are deep and you can feel them while others may describe a loose hatch as a knock.

Where exactly is the noise coming from? (This is also hard to determine. I have many hundreds of hours from the automotive world and can diagnose sounds pretty good there but on my boat it is very difficult for me to tell where it is coming from and what it means. Even squealing belts sound different.)

Did you lift the engine cover while it was making the noise? Is it from the front or the back of the engine, or maybe it is from between the drive and the engine? Can you feel it? Does it change with the engine speed? Does it sound like it is low in the engine block? You may have to remove the engine cover and have a friend help record the symptoms at speed.

When was the last tuneup, has the timing been checked, etc.

 

[Hansolo99]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

I have had the boat for about 4 months and this was the first time I took it out and really opened it up. Fresh oil and lower drive lube. The sound does not effect the speed, temp or oil pressure and I cannot really feel it. At first I thought it was just the water on the bottom of the boat, but after noticing the Tach I saw it drop and then jump about 100 RPM.

The jumping of the Tach was not a steady (every 3 seconds) instead it was intermitent (1 sec., then 3 sec. later, then 2 sec. later etc, but steady). It is not the latch and does not sound so far off to be the outdrive, but instead sounds like the upper part of the motor, but with 2 small kids I did not open the hatch and have them hear for anything especially at 42 MPH. Thanks in advance guys......

 

[Maclin]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

More info is always good. I was trying to help you undestand why no one had responded yet, there was not much to go on from the original post. I knew it was not a latch, just trying to help you understand how blind and deaf we at this end are in this situation. And I am happy you did not make the kids listen to the uncovered engine! THAT would have been a Homer Simpson move for sure

When was the last tuneup? Was the timing checked per the book procedure?

Knocking is a bad sound to me, means someting deep inside like rod bearings or worse. Tapping is better, but still could be valve/lifter problems. I believe your engine has an electric fuel pump, mechanical pumps can tick and tack and clack but this is not your problem. It can be one of the accessories or something driven off of the engine belt(s). It could be pinging from improper timing or other ignition or fuel delivery problem.

The tach jumping around says electrical to me, but again without hearing the noise would not know if this is a symptom or just something that would happen then anyway.

So the tach is steady until then? And you have never had it above that rpm? Is the sound bad enough that you feel something bad would happen if you tried to hit 4,000rpm?

 

[Hansolo99]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

Well it does not sound bad like something major would happen just too worried to try and hit over that RPM. I do not think it was tuned up at all just regular oil and lower unit fluid changes. The Tach is steady up until that point and if I back off the throttle just slightly to 3600 RPM the sound goes away then if I throttle up to 3700 RPM that sound is there again I thought maybe it would be timing, but was not too sure as I was told at the River by a boat mechanic (although it was just someone I met not at a shop) that if it were timing it would always do that? Not sure at this point, just thought I would ask some of the guys out there.....

 

[Maclin]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

If the distributor is working right regarding the centrifugal advance then the mechanic is right, the pinging and knocking would be a gradually worse thing and not as you describe the noise you hear and when you hear it.

I guess I would "had to have been there" to get more ideas.

 

[Hansolo99]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

Well thanks! I am going to take it in maybe in the next few weeks and have it looked at and I will update what they find.

 

[Silvertip]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

The 4.3 V6 is a balance shaft motor and many of those engines develop what can appear to be pre-ignition (noticable in a car if you know what to listen for). The problem is the bearings for the balance shaft have a tad too much clearance. IF this is the case with your engine and it appears to me it is, then don't fret. The engine will live a long and normal life and the tach issue has nothing to do with it. I've owned a bunch of 4.3 V6 powered vehicles - some made the noise, some didn't and they all had 150,000 miles on them with no issues. If the noise gets progressively worse then it is not likely the balance shaft and will need further diagnosis.

 

[Hansolo99]

Re: 2002 4.3 V6 IO Merc 3700 RPM knock sound? What is it??

Could it be the ignition module in the distributer? I had a a 1991 Dually with a 454 and the Tach sould jump at 2400 RPM. Just a thought and maybe some input?