190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

Killerkev

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Jun 17, 2011
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I have a 1986 Mercruiser 190 3.7L 224 CI engine and an Alpha one Stern drive. I am experiencing a loud knocking/rattling when the boat is under load. Now for some background history.... This year had the boat in the water, everything ran just fine and dandy. I was taking in water so the next day I looked at the bellows and there was a nice large chunk of the universal bellows missing. Took the boat out of service and proceeded to change all the bellows and gimbal bearing. The drive shaft on the drive was pretty rusted so I had it replaced by a marine mechanic along with a seal since there was gear lube all over in the bellows. Put it all back together, installed the drive and made an adjustment to the shift cable according to the manual.

With fresh gear lube in the drive, I started her up in the driveway and put it in both forward and reverse. All sounded just fine.

Brought the boat to the dock. Puttered out and slowly and began to increase the throttle. Before I even got up on plane a loud knocking sound occured. I backed off and slowly did the same. Again a loud knocking noise. Stopped and puttered back to dock. Attached the muff once out of the water and I can hear the noise, only slightly at idle now. No noise when slowly increasing the throttle through the whole range. Can only duplicate the noise (to some degree) by whacking the throttle when out of the water. Took a mechanics stethoscope during idle and listened to both bottom end, upper end, rear...etc..All sounds just fine EXCEPT the transom plate on the rear of the boat. This is the only place I hear slight knocking at idle. Have not had the opportunity to have another person whack the trottle while I listen with the stethoscope.

Now before I picked up the stethoscope, I removed the stern drive and ran the motor (with cooling water) to try to isolate if it was due to the drive or the motor. The noise WAS STILL THERE. Again only noticable when whacking the throttle. I had re-installed the drive for ease of supplying cooling water and began listening with the stethoscope as mentioned above.

So here I am, Boat was just fine running as it should first day of the season, find torn bellows, replace them and gimbal bearing and universal on the drive, reinstall and now a loud knocking coming from the motor when under load.
I should mention that I did purchace a alignment tool and was able to insert it fully to the back of the coupler. Although the markings on the top of the tool indicated that the front of the motor is high, but the front motor mount is crancked all the way down. I was able to easily insert the drive shaft into the coupler.

The motor was rebuilt and has only approx. 70 hrs. on it. It was pulled last time due to a spun bearing. Don?t recall if it was a main or a rod bearing.

So what do you think?.............
 

dubs283

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

take your stethescope inside the boat and locate the noise from the engine

with the motor running, can you see if the coupler/flywheel is wobbling?? (looking through the gimble bearing)
 

Killerkev

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

take your stethescope inside the boat and locate the noise from the engine

with the motor running, can you see if the coupler/flywheel is wobbling?? (looking through the gimble bearing)

Well, the problem is reproducing the noise OUT of the water. Can only do this by whacking the throttle. I brought the boat to my marine mechanic this morning for him to give a listen. He said that it sounded like it was backfiring... He suggested changing the water seperator, overall tune up..points, plugs cap, etc.. get rid of the gas, put some 93 octane in it...I thought well hell, that would be grand if it was that.......Did it all today and still the noise is there...even more now. My neighbor stated that it sounded better before I did the tune up! I agreed. I now may be able to take the stethoscope and determine something since it seems that the noise is more pronounced now since I've done the "tune up". Thinking of taking it to the dock, leaving it on the trailer and running it to create the load on the whole system. This way I can pull ignition wires to see if I hear a difference in noise.

After I do this, if it comes down to it I could easily pull the drive and run the motor and take a look at the coupler to see if she is wobbling. But as I stated earlier, I had taken the drive out and ran the motor and still had the noise. What are you thinking? Loose bolts holding the coupler?
 

dubs283

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

What are you thinking? Loose bolts holding the coupler?

its possible, or loose flywheel bolts

when you did the tune-up did you set dwell and timing to spec??
 

stonyloam

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

said that it sounded like it was backfiring...

I wonder??? There is a little casting plug in the back of the head that sometimes rusts out and dumps the #4 cylinder exhaust directly out of the head, can sound like a knock or backfire. Put your hand (carefully) back of the head and see if you can feel bursts of exhaust coming out. Just a guess.
 

Killerkev

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

its possible, or loose flywheel bolts

when you did the tune-up did you set dwell and timing to spec??

Yes, I set everything according to the manual/spec. I think that it's sounding worst due to what every my problem is. I did not touch it today because I needed a break from it. The other day it was rubbling more than it has...I could hear the slight noise at idle where before it wasn't noticable at all. Going to take some time tomorrow after work. Gonna check compression. Hopefully I can take it to the dock and pull individual wires to see what happens. Trying to do all before I have to pull the motor. My gut is telling me that its a rod bearing....I want to be wrong.

Also, When I started it up after the tune up... as it cranked, there was a bang noise. I did have this happen before since this problem came up. Sometimes when starting it it will bang other times it doesn't.
 

Pete104

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

Try the stethoscope at the front of the engine also. Put it something solid that isn't moving with the engine running.
 

Killerkev

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

I checked that freeze plugand it's still there. Reset the dwell and timing and it is running better now. Had some help for a short time today, he also thinks it sounds like backfire. Maybe a valve isn't closing or burnt. Hopefully tomorrow I will have a good portion of the late afternoon to do more troubleshooting. Any suggestions what to try to pinpoint a leaking valve problem?
 

ericcarr2000

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

I agree, do a compression test on her. That will tell you if you have a stuck or bent valve train
I checked that freeze plugand it's still there. Reset the dwell and timing and it is running better now. Had some help for a short time today, he also thinks it sounds like backfire. Maybe a valve isn't closing or burnt. Hopefully tomorrow I will have a good portion of the late afternoon to do more troubleshooting. Any suggestions what to try to pinpoint a leaking valve problem?
 

Killerkev

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

compression test and leakdown test

Got the engine warmed up and took all the spark plugs out and did a compression test. All cylinders ranged from 120 to 130 psi. Hooked up a vacuum guage to the intake manifold, reading low I believe, a steady 13 psi. Sprayed WD40 around intake manifold gasket and carb gasket...no change a steady 13 psi.

Checked the timing and point gap again. Even brought cylinder 1 up to TDC and checked the distributer cam... all right on.

I'm at a lose here. Last thing is to put it in the water, get it under load where it constanly makes the banging noise and start pulling ignition wires. After that if something doesn't rear it's ugly head then I'm screwed and may have to dump $$$ for a profesional diagnosis......UGH....
 

Pete104

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

Listen to the front of the engine. Behind the balancer. The crank gear on "alot" of these engines loosen up. Even though the "chain tensioner" stayed in spec. However, if that wore out or broke, you will still get a "hard noise".
 

Killerkev

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

Well....finally got the boat in the water and tied her to the dock. Put her in gear with a knowledgable helper on board and increased the throttle until the "noise" made it's self know. Definately coming from the front of the motor. We agreed sounded like from the timing chain.....

A few days later and another $150 in parts (by the way, the chain tensioner is no longer available....eBay), began prepping the front of the motor to take the timing chain cover off. Pulled the harmonic balancer and found that the balancer was hitting the front motor mount! My mounts are shot! What a long trip this has been! Ordered new mounts all around (another $120) waiting for them to come next week.

It all makes sense now. Boat ran fine fist day of spring in the water except taking in water. Changed bellows etc... Test her in the water... noise coming from motor. But I didn't touch the motor..what the hell? But I DID touch the motor....when I attempted to make adjustment to the front motor mount for drive alignment purposes...I didn't think that I made any adjustment since I thought it was all the way down...I must have made a slight adjustment because now the balancer was hitting the mount.

Man, I've rebuilt, repaired sooo many items on this boat. Wondering what will fail next year? There isn't much left that I haven't repaired...well, there are a few items but I don't want to jinx myself.....
 

dubs283

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

Man, I've rebuilt, repaired sooo many items on this boat. Wondering what will fail next year? There isn't much left that I haven't repaired...well, there are a few items but I don't want to jinx myself.....

get used to repairing your boat, its not a matter of if, but when items fail

routine maintenance is your best friend with boating - keep up with it, you will be glad you did

unfortunately for you your engine is probably one of the worst mercruiser ever made and tend to be maintenance hogs
 

stonyloam

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Re: 190 HP Mercruiser - Sudden Knocking noise in motor after bellows repair

unfortunately for you your engine is probably one of the worst mercruiser ever made and tend to be maintenance hogs

They do keep you on your toes, but they don't crack LOL.
 
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