Knocking noise on 7.4l Bravo III

M

mjfrank

New Member
Jun 2, 2010
42
Lake of the Ozarks
Boat Info
1998 280 Bowrider
Engines
7.4L Merc, Bravo III
Hey all:

Our "new to us" 98 280 BR just gave us a little scare this weekend. We were out tubing with the kids on Saturday, and had been out with the boat for most of the day. It was running great, and the only issue that I noticed was that the throttle/shift control didn't seem to be adjusted correctly, as a couple of times the handle was in the neutral detent, but I was still in forward gear. I had to jiggle the handle back a couple times to get it to disengage.

The kids started waving that they were done tubing, so I motored off to the side of the cove, and pulled the boat back to idle (still in gear), and it suddenly started making a loud rhythmic clunking noise from the engine/out drive area. (loud enough for my wife to notice, which is saying something) I put it in neutral and immediately shut it down, thinking that maybe the drive belt jumped off or something was tangled in the props, because that is almost what it sounded like. I checked the props, nothing, and then opened the engine hatch, and the belt was fine. So now I'm thinking I threw a rod in the engine, so I had my wife crank it up while I listened. The engine cranked up fine, good oil pressure, temps right at 170. By leaning my ear near it, I could tell it was coming from behind the engine.

I had her shut it down again, while I contemplated choking someone at the boat dealer. My wife started looking for the dealer's phone number, and I decided that rather than drift into the rocky shore, I should at least try to idle over near someones dock and tie up. So I cranked it again, still making the noise, put it in reverse, still making the noise but moving, back to neutral, still clunking, put it in forward, and suddenly NO NOISE! Back to neutral, no noise, put it in reverse, no noise, forward again, no noise, shutdown and restart, still no noise.

So I ran it back to our dock, and it was fine the whole way. We also took it out Sunday, and went about 15 miles up and down the lake, with not a single issue. (other than the control is still not working right)

The boat has 470 hours, and the dealer just replaced the gimbal bearings(engine was pulled), trans shifter cable, u-joint and exhaust bellows, and some seals on the lower unit. I have a thirty-day warranty until July 12th, so when I dropped it at the dealer's marina, I asked them to pull it into service for them to adjust the control and investigate the noise I heard.

The only thing I could think of was maybe a coupler problem (but no burnt smell from the engine compartment, and no loss of power), or a problem with the clutch in the out drive? The drive oil level is fine, and I didn't get any warning horns.

Does anyone have any ideas on what could have caused the noise, and where I should tell the mechanics to look? We are going back down this coming Sunday, so I'm hoping they can do something before then.

Thanks!
--Mike
 
Hope you get an answer to this one, very strange it went away.:huh:...
 
Yeah, I'm hoping you guys are right, and it was just the trans cable being out of adjustment. I was going to attempt to adjust it myself, but since its still under a dealer warranty, I didn't want to jeopardize that in case it was something more serious. (plus, I don't have that little adjuster tool doohickey)

I'm hoping they get the cable adjusted by Sunday, if not, they will encounter the wrath of my wife when her weekend plans get ruined.:smt013 :smt021 God help their souls. :grin:
 
After you get the adjustment.......

I was told by 2 experienced guys, not to try & ease the drive into gear but firmly shift fully into gear. I have heard the knocking you speak of if I try to "baby it".
 
I had that same problem on a mercury outboard and it was the gears popping in and out down in the foot of the bottom of the outboard. The shifter cable had play in it and wouldnt fully engage the gears. Simple adjustment and problem solved.
 
If you backed down to fast you may have knocked the flapper values in the exhaust system out of alignment. I have done this with a 7.4L engine and it sounds like the top end of the engine is knocking. Once you get some water running through it they go back into place.
At heavy throttle there is a lot of water flowing through the system, when you back down to fast the water has no where t o go.
 
mwph said:
After you get the adjustment.......

I was told by 2 experienced guys, not to try & ease the drive into gear but firmly shift fully into gear. I have heard the knocking you speak of if I try to "baby it".
Click to expand...


I was told the same.....Firm is the way....:thumbsup:
 
Hmm, all good things to make a note of.

I'm pretty use to making sure I do a firm shift into and out of gear, my other two boats both have outboards (Johnson 140hp 4-stroke, and Merc 40hp 2-stroke), and if you linger even a little bit while shifting, you get bunch of grinding. I make sure it's one fluid movement in and out (well actually "out" is a little tough right now due to the adjustment problem)

I guess it might have been the exhaust shutters(flappers). They were just replaced, so they should be in good shape. I didn't really drop the speed that quickly, it was a fairly gradual slow-down to idle, which was when the noise started. I'll keep that in mind if it happens again though.

I'll let you know if the mechanics find any other oddities after they adjust the cable.

Thanks again!
--Mike
 
Last edited:
mjfrank said:
Hmm, all good things to make a note of.

I'm pretty used to making sure I do a firm shift into and out of gear, my other two boats both have outboards (Johnson 140hp 4-stroke, and Merc 40hp 2-stroke), and if you linger even a little bit while shifting, you get bunch of grinding. I make sure it's one fluid movement in and out (well actually "out" is a little tough right now due to the adjustment problem)

I guess it might have been the exhaust shutters(flappers). They were just replaced, so they should be in good shape. I didn't really drop the speed that quickly, it was a fairly gradual slow-down to idle, which was when the noise started. I'll keep that in mind if it happens again though.

I'll let you know if the mechanics find any other oddities after they adjust the cable.

Thanks again!
--Mike
Click to expand...


Keep us posted mike.....:thumbsup:
 
Ah, the saga continues. After having no problems with the boat on the July 4th weekend, and putting about 10 hours since we last heard the noise, it reared it's ugly head again this weekend.

We were down moving the boat to our slip, after two months in a marina. She ran fine Friday night on the way over to the dock (about 5 miles), and was fine for the same 5 miles back up the lake to get gas on Saturday. We were heading up about 3 miles from the marina, when my wife yelled that the boat seemed louder than normal, which I noticed too when I leaned down behind the windshield. I got over to a safe spot and pulled it to idle, where I could once again hear the knocking from the stern. The sound is there in neutral or in gear, the frequency goes up and down with engine speed, and doesn't change when trimming up or down or turning left or right. All the time, the boat was performing normally, no loss of speed, plenty of drive oil, no overheating, etc. The only difference this time, was that the sound was slightly quieter.

So I figured the hell with it, I've got to get it back to the marina, so I ran it back at reduced speed, making the noise all the way. (almost sounded like a diesel rattle) I made it back and they sent a mechanic down to give it a listen. Sitting on the dock with the engine running in neutral, you could tell it was coming from the upper gearcase area. I put my hand on the top gearcase cover, and you could feel the knock. So, we left it in their hands to figure it out. (and had to hitch a ride back to our dock from the marina owner)

The last time I talked to them today, they had the outdrive off, but had yet to tear into it. The things that they "claim" to have replaced before we bought it were the gimbal bearings, the "back-door" seal (I don't know what that is), the shift cable, the shaft, cable and exhaust bellows, and the engine coupling. No mention of replacing the u-joints, which I'm thinking might have been damaged if the shaft bellows were leaking. The engine seems solid, and they said it wasn't knocking with the outdrive removed.

So, what do you think, u-joints, gear lost a tooth in the upper gearcase, etc? Do Bravo-3 drives have a clutch in the upper case, and could that be bad? One other possibility that I got from searching the web, was failed rear engine mounts? They did pull the engine when the coupling was replaced, so maybe something was damaged when it went back in.

Hopefully I'll get a better idea when they crack open the gearcase tomorrow. I'm just hoping it's something simple (-ish), or they eat the cost on it. (I'm sure not without a bunch of arguing on that latter one.) We had a warranty, and the noise was reported while it was still in effect, but they couldn't find the problem then, and the noise went away.

--Mike
 
Thanks for the update Mike
And the saga continues....:smt009
 
Well, the shop just called, and it looks like a gear(s) lost some teeth, which then circulated metal all through the housing. So, basically a complete rebuild of the upper and lower cases. Total cost around 4500, the owner is willing to split it 50-50 since we've only had the boat for 2.5 months.

They say all new gear sets, bearings, and seals, at my cost of around 2200 to get back on the water. Does that sound like a decent deal. My only aggravation is that if it would have kept making the noise a month ago, they would be eating the whole cost of repair. But unfortunately it was only noisy once, and then it was quiet until last weekend.

I'm back to being kind of stuck, the boat is there and torn down, and it will cost me to try to haul it anywhere else. I'm pretty convinced to just pay the cost, and hopefully end the boating season on a good note. Anybody have any advice here?

--Mike
 
What kind of warranty are they willing to give you? $2200 is probably a good price based on what they are going to do but if it fails prematurely again and is not covered by a warranty then I am not so sure.

Dave
 
mjfrank said:
Well, the shop just called, and it looks like a gear(s) lost some teeth, which then circulated metal all through the housing. So, basically a complete rebuild of the upper and lower cases. Total cost around 4500, the owner is willing to split it 50-50 since we've only had the boat for 2.5 months.

They say all new gear sets, bearings, and seals, at my cost of around 2200 to get back on the water. Does that sound like a decent deal. My only aggravation is that if it would have kept making the noise a month ago, they would be eating the whole cost of repair. But unfortunately it was only noisy once, and then it was quiet until last weekend.

I'm back to being kind of stuck, the boat is there and torn down, and it will cost me to try to haul it anywhere else. I'm pretty convinced to just pay the cost, and hopefully end the boating season on a good note. Anybody have any advice here?

--Mike
Click to expand...

mike; you might want to check into a mercruiser factory re-built.
we found one for 3400. we did get a good deal as we bought the transom ass. and all other parts for the swap from the same dealer

you get pretty much a new drive with the same as new mercruiser warranty.

we bought one in june of 09 for our 230 when i swapped from the alpha to b-3 drive.
 
They are putting a 1 year parts and labor warranty on the work.

He said that is the same as a Mercruiser reman'ed warranty?
 
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